All for Myself

Ninnik Nishukan

Summary: Petunia Evans was playing with her food…and then she was playing with fire. Petunia, Lily and Snape. Pre-Philosopher's Stone AU.

Perhaps she was making Polyjuice Potion because (after only mixing cleaning solution for a couple of weeks) it sounded like a challenge, especially since it took so long to make, or maybe it was because she'd rationalised it by deciding she wouldn't be using it on herself, after all, but only on the cat and the dog.

Petunia promptly ignored the momentary, dizzying fantasy of turning herself into her sister and waltzing off to Hogwarts.

The reason why she chose that potion didn't matter, anyway. What mattered was that she was now standing in a cramped room that was rapidly being overtaken by misty, purple-hued smoke and watching a pot whose contents were moving jerkily, spasming, as if they were alive. If only she'd been out in the garden shed as usual, but it was far too cold, so she'd been forced to move her project to the bathroom.

Perhaps, she thought absently, dog hairs weren't suited for this potion, perhaps it could only be human hair, or perhaps that fluxweed hadn't been picked at full moon as advertised, or perhaps she hadn't powdered the bicorn horn finely enough, or perhaps she'd misunderstood what "unsucculating" the leeches meant, or perhaps she hadn't let it all sit long enough before she'd finally added the dog hairs—

She shook her head, staring at the bubbling pot, her skin crawling with the rising panic.

What if it blew up? What was she supposed to do?

Spinner's end, she thought wildly, feeling faint, that boy…!

He'd know!

Lily was with their parents, out Christmas gift shopping, from which Petunia had begged off by claiming to have caught a cold, but that boy…she knew he was home for the holidays as well, and there was no chance that he was out buying any gifts— surely not with a family like his, and not when he couldn't even afford any decent clothes!

But somehow, her feet weren't moving down the stairs and out the front door. She just couldn't ask for his help. She could practically hear his derisive laugh already. Petunia had enough self-awareness to know she wasn't exactly a ray of sunlight herself, but that horrible Snape boy…he was just cold. Cold, sharp and resentful. Everything about him— his eyes, his voice, his mannerisms, his clothes, his hair— was either offensive or vile, or both.

Her stomach twisting with anger at her own stupidity, she bolted to the small bathroom window and threw it open, a blast of cold, sharp air hitting her; she was suddenly not caring about the neighbours finding out. The room had become too unbearable.

As she was leaning across the window frame and gulping air, watching the purplish smoke spewing out of the window and floating upwards, she heard the sound of footsteps on a combination of gravel and hardened snow.

Her gaze dropping, she caught sight of the very boy she'd just been thinking of, lurking near the house, his hand resting tentatively on the picket fence gate. No doubt, he was there to see her.

Petunia drew another breath, desperately wanting to interrupt him before he asked her, in that dodgy, grumpy, not-quite-meeting-your-eyes sort of way he had, whether Lily was home. Now, in the hour of what she saw as her greatest failure since…since Hogwarts, she did not need to hear anything about Lily.

It was already enough to know that the loathsome boy, with his overlarge, abused old coat that made him look like a flapping bat when he ran (and she'd had ample opportunity to see him wince and run every time one of his parents yelled for him), braved staying with his wretched family each Christmas instead of remaining at Hogwarts just so he could see his beloved Lily over the holidays, like some sort of twisted parody of a prince.

He was quicker than her, however. "Don't tell me you're actually trying to do magic," he said bluntly, a spark of nervous anger behind the accusation as his eyes flicked between her and the purple smoke, which had started sputtering out tiny sparks by now. They reminded her of the sparklers she and Lily had played with at New Year's Eve as children, and looked altogether unnatural.

For a few seconds, she simply gaped at him, torn between feeling indignation at his condescending tone and relief from the fact that he hadn't mentioned Lily yet.

"I take it your sister isn't home, or I highly doubt you'd find yourself in this mess," he continued, however, in that snide tone of his that always made her skin itch. So much for keeping her sister out of the conversation for once.

"Oh, of course not," she spat, with acidic sarcasm, "or Lily Evans the Potions master would already be saving the day, wouldn't she?"

Petunia watched his eyebrows rise, and realised with some surprise that her poisonous comeback had actually impressed him a little. "If you let me in, I can fix this," he deadpanned. "Muggles shouldn't play around with things they couldn't possibly understand."

Sighing, she went downstairs to do just that, not finding the energy to bicker anymore. She was finally starting to worry about the neighbours.

Severus hadn't been lying. He could fix it. Even without magic, he could fix it.

It was almost annoying how easy he did it, too. He simply walked up to the smoking pot and dabbed a couple of drops of clear liquid into it from a tiny vial he'd pulled out of one of his mysterious, large coat pockets; the potion abruptly calming down, the smoke gradually evaporating.

"Usually I just use a simple vanishing spell," he explained, when he caught her flabbergasted look, "but since I can't use magic outside of school, I invented this little elixir so I could still practice my Potions during the summer."

Still staring at the now almost offensively serene pot, she only nodded.

Severus stroked his chin as he examined the pot; he looked ridiculous, she thought, like he was trying to imitate a clever old man. "Hmmm…judging by the look of this horrid sludge, I'd say you were attempting to make Polyjuice Potion," he sneered, almost more at the pot rather than at her, as he gave the wooden stirring spoon a guarded poke; it barely moved, only producing a disgusting, wet sucking noise from the depths of the pot, "and judging by the putrid smell, and the amount and colour of the smoke, I'd say you not only took a wrong turn somewhere with the instructions, but that you've also managed to get some sort of animal hair mixed up in it…I've seen it happen dozens of times, and I keep wondering why people insist on bringing their little pests to school…" he added in a mutter, hunching over and glowering at the brown-green gloop in the pot.

Meekly, she cleared her throat. "You're not…supposed to…?"

Straightening his back, he sent her a nearly pitying look. "Oh, but you actually intended to put that animal hair in…didn't you?" he said with feigned sadness, shaking his head at her.

Petunia's face felt ablaze with embarrassment. "The recipe didn't say, so how was I supposed to know?" she spat, itching to throttle this nasty, rude boy who came into her house in those rags, and was younger than her, yet acted as if he was an adult and she was some sort of disobedient child

Then she noticed him starting to take in the other interesting objects of the room. She had to squash down the abrupt urge to lunge forward like a small girl trying to grab her most embarrassing diary from a local bully. It was silly, she knew, but she liked to have her finished potions nearby when she was working, so she could look at them for inspiration.

"Well, well, well, it looks like this wasn't your first attempt, after all," Severus goaded as he perused the small wicker baskets in which she'd lovingly nestled her potions among soft, shredded napkins, like fragile Easter eggs. "Of course, I already had a suspicion you were up to something when I met you at Slug & Jigger's…"

"Yes, yes, very amusing," she said, stepping up to him and getting ready to pull him out the door by his arm if she had to. "Listen, I do appreciate your help, but really, I think it's time for you to leave…"

Severus' eyes narrowed as he took a much closer look at her experiments. "You know, these are actually…"

Petunia steeled herself, lifting her chin defiantly as she waited for the withering look and the harsh judgement.

"Not bad."

She blinked, almost startled. "What?"

"Yes," he drawled, his long, thin fingers curling around the flask she'd poured the Elixir to Induce Euphoria into, "some of these could clearly do with a bit of work, but they're still better than some of the foul slop most of my classmates cook up…I mean, at least the colour of this looks exactly right."

"Sunshine yellow," she heard herself murmur, dazed by the concept of being deemed better at anything than anyone at that wretched school, even if the evaluation was coming from Severus Snape.

"Yes, and," he went on, unstoppering the flask and sniffing at its contents, before putting the stopper back, "the scent seems right as well. You remembered the peppermint. If nothing else, you're at least capable of occasionally following instructions properly." His gaze darted up to meet hers, and seemed to be scanning her intently. "Have you tried any of these?"

She swallowed, wondering if he'd be able to tell she'd been too afraid to try some of them. "I've…not all of them. Some I just made to see if I could. I've mostly just been using the magical cleaning fluids and the Murtlap essence and…uh…" she paused, her face slowly turning red, "…the Boil-cure Potion."

This, however, didn't seem to faze him; she suspected he'd probably had a few boils himself at one point or another. He also hadn't commented on the heavy rubber apron she was wearing over her yellow blouse and embroidered blue jeans, or the rubber boots and gloves, or the frizzy, dishevelled state of her normally curly blonde hair; not that he had any right to criticise her, considering his own jeans looked ancient, his black boots and charcoal-coloured jumper and coat were not only too big, but too thin-looking for winter, and his hair was as lank as ever. Nevertheless, she'd taken the time to at least remove her goggles and run a glove-covered hand through her hair before answering the door.

"And these potions worked?" he prodded, sounding intrigued now.

Her chin rose again; this time with self-satisfaction. "They did what the book said they would, yes."

Severus chuckled briefly; coming from him, it was a foreign sound. "Would that book be one of Lily's?"

"Who else's?" she answered at once, refusing to feel embarrassed again. He could already tell she'd been sneaky about it, and he didn't seem to have any objections.

"The instructions for Elixir to Induce Euphoria are written in Advanced Potion-Making by Libatius Borage, and we're not using that book until sixth year…not to mention Polyjuice Potion, which, to my knowledge, can only be found in Moste Potente Potions, which can in turn only be found in the restricted section at the Hogwarts library or in other wizarding libraries," he remarked with a deceptive kind of casualness, glancing sidelong at her, "so where did you get them?"

So that was what 'Adv. P.' had meant, Petunia thought, once again struck by the discovery that she'd done better than she'd thought despite her failure du jour— Advanced Potions! Advanced! Perhaps that was why the clerk at the Apothecary hadn't questioned her ingredient list, too; because she'd been old enough to be a sixth year. Briefly, she wondered if Severus knew who 'Sluggy' was, but didn't dare ask.

"There was a note in the Jigger-whatsit book," she said, shrugging, "she must've asked an older student about it and copied them down, I don't know…ask her, not me." She's probably trying to get ahead, overachieving just to get even more attention, she added sullenly to herself, pursing her lips for a second as she then toyed with the thought of mentioning the fact that his name had been on Lily's note, dismissing it quickly as she realised it'd only give the pitifully smitten boy even more false hope.

Severus sent her a lingering look of appraisal, breaking it only when she began to feel discomposed. He then picked up another bottle; this one smaller, its contents a lovely, pale blue. "You haven't been tempted to use the Wit-sharpening Potion yet, have you?" he queried, showing her a sly smirk.

"Ignoring that pathetic attempt at an insult," she replied smoothly, secretly pleased with herself for it, "using the potion would be cheating, as I assume that's what you wizards and witches use it for at school."

"Then why did you make it?" he shot back.

"We had some extra ginger root lying around and I reckoned I could try it," she explained, again pleased with how careless and blasé she sounded. "You're welcome to it if you need to pass your exams," she added flatly, biting back a grin.

"Well, aren't we droll?" One dark eyebrow arched for a second. "Despite your obvious ignorance of the wizarding world, you are aware I could report you for this, aren't you?" He tilted his head at her, the look in his eyes calculating. "I should report you for this, really. Or," he volunteered, in a silky tone, "I could simply tell your sister."

Petunia almost shuddered at the jolt of fear and resentment that slashed through her body, but then she surprised both herself and him: "I suppose you could…so why aren't you going to?" she challenged coolly.

He was staring at her now; apparently, he hadn't expected her to look through his idle threat, thus rejecting his little show of power over her. "Perhaps it simply holds some fascination for me…like an accident waiting to happen," he taunted, drawing himself up. "Perhaps it's not such a terrible idea to let you make an example out of yourself, so other silly little Muggles won't meddle in affairs they don't understand."

Petunia glared at him; only minutes ago, he'd been praising her, albeit in a reluctant sort of way. He'd clearly been impressed (whether over the sheer nerve of her or whether he thought she had real talent, she wasn't sure), but was now backtracking, trying to hide it. Or perhaps, now that he thought about it, he wasn't too keen on any Muggles imposing on his turf; Lily had always told her 'Sev' had a great talent for Potions. The best thing, though, she supposed, would be to ignore it so he really didn't get provoked into tattling on her. Swallowing down her biting retort, she tried to focus, rather, on the fact that he'd come to her aid.

"Thanks for the help," she gritted out, handing him the Elixir to Induce Euphoria and the Wit-sharpening Potion. "You'd honestly do me another favour if you'd take these off my hands, though."

He frowned at her, looking somewhat confused; it was the first time she'd ever seen that expression on him. Maybe he wasn't used to being given things, or being told thanks. "Really, I'm not going to use them, anyway," she insisted, and this he seemed to understand.

Giving her a curt nod, he slipped the small bottles into the spacious pockets on his overlarge coat. "I'll just find my own way out, shall I?" he said breezily, not waiting for an answer before he left.

Before your parents come home, seemed to be the unspoken statement.

While they hadn't made any forbidding rules as of yet, Petunia could tell her parents weren't thrilled over the concept of their daughters spending time with the scruffy-looking wizard boy from Spinner's End.

It was the summer before Lily's sixth year at Hogwarts, since her whole family had taken to describing time that way, and Petunia was haunting the playground down the street to evade her sister's gloomy mood. She'd never seen her like that before. Whenever she referred to him now, if at all, she said 'that boy' or simply 'Snape', as if that would distance them further; her tone uncharacteristically sour.

"My sister won't speak to you," Petunia said, the moment she saw him approach her where she was resting on the grass, "so it's no use asking me." Part of her secretly felt less hostile towards him simply because he had indeed insulted her sister, whom everybody usually couldn't stop complimenting…even as part of her couldn't help despising him for causing the normally sunny girl to cry. Why did family have to be such an inescapable and complicated thing?

The boy looked stung, but then he appeared to pull himself together. "Good, because that's none of your business, anyway. That's between me and Lily."

"She said you called her a very rude word," Petunia said, ignoring his warning and adding bluntly: "Mudblood. Is that like a stupid wizard curse or something?"

"That's not for Muggles like you to concern yourself with," he said, his voice tense and carrying a definite sense of finality. It seemed it was a subject he wished to avoid, which only made her want to pursue it even further.

"At least Muggles have manners," she returned haughtily, her nose wrinkling, "I'm sure I'd never call somebody anything like that."

Severus' teeth bared for a second, his lip curling in disgust. "No, you'd only call them freaks…which, by the way, is also a rude word in the wizarding world."

Petunia set her jaw. "Yes, well…it must've been a lot worse than what I called you, because you made her cry."

For a brief moment, he looked devastated. Then his features returned to their cold neutrality. He looked like he was going to say something insulting, but surprised her. "That was never my intention, I assure you," he said, looking absentmindedly out across the playground, and she was struck by how adult he'd sounded for a moment; which was unfair, considering he was younger than her.

She blinked at him when he suddenly turned his attention back to her. "So…how are your little adventures in potion making going?"

Petunia spent a few seconds trying to decide whether he was ridiculing her or not. Finally, she decided to ask him the humiliating question she hadn't been able to bring herself to ask Lily yet. "Severus," she began, and could tell she'd instantly caught his interest merely by using his actual name, "is there such a thing as…as a scholarship for Muggles?"

He gave her an odd look. "How d'you mean?"

Shrugging, she trained her gaze on her shoes. "I just…if I can make potions, if I even showed some talent…would there be a possibility for me to just…do that? Even if I can't do proper magic?" She looked up again, eyes imploring him. "Couldn't I still go to that school? Part-time or something? Or do something else in the magical world?"

Slowly, he shook his head, appearing momentarily bewildered. "I doubt that would work out. There are so many things that can go wrong with potions, as I'd assumed you'd have already learned by now," he started chastising her, but next his tone softened just a bit, making her wonder, with acute humiliation, whether she looked vulnerable. "If you can't do magic," he continued, his guarded, strict expression turning a shade concerned, "then there's no way for you to protect or heal yourself if something goes wrong, or get rid of the failed potions without damaging something."

"Can't I just use that clear elixir you used that time?"

"That's not always enough, that only works in some cases…and besides, you haven't got any training or any magical education."

"But if I had some supervision, if somebody could just…" she bargained, her sentence trailing off once she registered how pathetic she sounded.

Severus gave her a long, piercing look. "You applied for Hogwarts," he said; it wasn't really a question.

A painful lump started building in her throat. "Lily told you, didn't she?" Petunia demanded bitterly, her voice sounding much smaller than she would've liked.

"She did no such thing," Severus objected, scowling at her. "It doesn't take the deduction skills of Sherlock Holmes to see that you're jealous of her…because you wanted to go there, too."

The words describing her secret shame were out there, and they were awful. But not as awful as she'd imagined they'd be. "I suppose you find me silly," she pressed out, one hand clawing at the grass.

He shook his head, and then he was actually sitting down next to her on the grass. "Mostly uninformed and hopeful, I'd say."

Petunia was quiet for a long while. When she finally spoke, she sounded calmer. "Speaking of uninformed…I've been wondering…where does Abracadabra come from? Does that mean anything in your world? Would anything happen if somebody with magic said it?"

A fleeting, mocking grin went across his sallow visage. "A rabbit out of a hat, you mean?"

Pulling a withering face at him, she then rolled her eyes. "Never mind."

"You'd have to ask a professor of magical linguistics," he suggested, and she was puzzled to hear he seemed to actually be giving the foolish idea some semi-serious consideration. "Maybe it's something dating far back, something that's lost its meaning now but has somehow survived as an expression in the non-magical world."

For a moment, she wondered if he was simply hungry for conversation. As she studied the greasy-haired, hook-nosed, pale-faced boy, she imagined he hadn't got many friends at that school. Suddenly, her hands itched to wash him, which frankly disturbed her, even if she'd obviously meant it in a non-sexual way. She wondered whether her obsession with cleaning, as much as she enjoyed it, could be a little unhealthy at times.

She still wondered whether he'd be offended if she asked if she could press him to some shampoo, and then wondered if she even cared if he'd be offended or not.

"I said never mind," she said lightly, getting up and brushing the grass off of her new and delicately flower-patterned, blue summer frock. "I have to get home and cook supper, anyway," she informed him, not quite friendly, but far from dismissive. "And you should go home and mend your jumper…there's a great big hole right there on the elbow where everyone can see it."

The boy remained sitting, his expression indecipherable, but his cheeks faintly red. It seemed her last remark had affected him badly, and he didn't want her to know his humiliation.

Licking her dry lips, she shifted impatiently, lingering. "I expect I'll see you later," she prompted, when he didn't speak.

"I'm forced to stay at this miserable place until September first as usual, yes," he said, in the same non-committal tone she'd used.

Nodding, she walked home.

Severus was back for the sixth year Christmas holidays, just like Lily, and he actually came right up to their door instead of skulking around outside. When Petunia tip-toed downstairs, they were deep in a hushed, but heated conversation. From the living room, she could hear him murmuring back and forth with her sister on the doorstep.

Petunia only sat still on the sofa and kept pretending to read her book, gradually leaning towards the door until her side hit the arm rest, her ears straining.

"…but won't your dear Death Eater friends disapprove of what you've done, once they find out?" she heard her sister hissing reproachfully, "Won't they accuse you of 'contaminating' the 'pure' magical world or something?"

Petunia frowned; she'd have to ask Lily about 'Death Eaters' later. They way she'd said it, it sounded like some sort of club for a questionable sort of people. In Petunia's views, though, 'questionable' included a lot of people, so it could mean anything.

Next, she could practically hear the boy squirming uneasily; could picture his sallow face, contorted in an awkward grimace. "Yes, well…I shall have to cross that bridge later, if they find out…"

There came an exasperated sigh from Lily. "Oh, Sev…"

"Sorry," she thought she heard the boy say, then, and he sounded shy, almost subdued; yet there was an undertone of frustration. "Lily, I really am sorry."

"Just don't ever say that word again," she heard her sister reply, her voice louder and harder, but not entirely unforgiving.

"Never," he muttered; Petunia thought she heard him shuffling his feet. She suppressed a snort; served the git right, being all nervous. Petunia had questioned Lily further about 'Mudblood', and discovered it was a very bad word indeed. It seemed that some wizards and witches viewed people like Lily as barely above Muggles, who they in turn viewed as worthless. Yet Petunia still wasn't quite certain how to feel about that; offended on her sister's behalf, or somewhat gratified on her own.

When Lily came back into the living room, she was carrying a tiny wrapped Christmas gift, smiling softly to herself. Sitting up, Petunia realised she still hadn't heard the front door close. Discarding her book, she stood up from the sofa.

Just as she opened her mouth to speak, to ask, Lily beat her to it. "Sev wants a word with you," she said, grinning with conspiratorial glee at Petunia, as if there was something amusing about that.

Petunia clucked her tongue disapprovingly. "Oh, so now it's 'Sev' again, is it? I thought you'd decided you two weren't friends any longer?"

Lily groaned, looking faintly uncomfortable. "Come off it, Tuney…just go and hear what he has to say, all right?"

Rolling her eyes to hide her curiosity, Petunia walked out to meet him at the door. Leaning against the door frame, she gave Severus a sceptical look, scrutinizing his usual old, large coat, but new-looking, long silver-and-green scarf and equally unfamiliar black woollen hat that was covering up his miserable hair. "I'm glad you've said you're sorry, because even my little sister deserves an apology," she said, scowling at him, "but I hope you don't think it means you're anything more than friends now just because you're forgiven, because that's a sad delusion that'll only end in—"

"There's just so much you feel the need to comment on that's really none of your business," Severus interrupted coldly, "I expect you'll evolve into the neighbourhood gossip in whatever tedious suburb you move to in future."

"Lily said you had something to tell me," Petunia said, projecting impatience at him with everything she had.

"Now that I've met you and your charming self in person again, I've no idea what ever possessed me to do what I did," Severus said, icy disdain permeating every syllable.

"Which was what?" she prodded, her curiosity getting the better of her again.

Severus released a heavy sigh. "Against my better judgement, I passed your potions on to Dumbledore, along with your name."

It took a moment before Petunia remembered how to speak. "But hadn't they gone a bit off by now? It's been over a year."

"Oh, potions can keep quite well…if you know how to keep them," Severus said enigmatically, unable to keep a hint of pride out of his voice.

"And what did he say?" Petunia refused to acknowledge the fact that her voice was actually shaking now.

"It turns out I was right, and there really is no such thing as a scholarship for Muggles, so you still won't be able to attend Hogwarts, even for Potions," Severus said, apparently not realizing that her stomach was plummeting to her feet with the crushing disappointment, because he only went on: "However, it appears that for some reason, Dumbledore has taken an interest in you, and wants to arrange a meeting between the two of you and the current Potions master, Professor Slughorn. It seems he feels you've got some talent for precision and details," he volunteered grudgingly, before sniffing and adding: "If you ask me, Dumbledore probably just wants to make certain you'll be prevented from causing any real trouble, but…either way, he still wants to set an appointment with you. I expect he'll send you an owl soon with a formal invitation."

Now, Petunia seemed to have forgot how to breathe. Struggling with the overpowering feeling of surrealism, she wondered dreamily whether this 'professor Slughorn' was Lily's 'Sluggy'. When she finally drew a breath, the first word she blurted out was her sister's name. She couldn't help it.

"LILY!" she bellowed, her heartbeat thundering along with her footsteps as she ran inside the house to tell her sister the news.

It wasn't until that evening that it occurred to Petunia that even if Lily seemed not to know, Severus must've already told Lily, because what Severus had done for Lily's sister must've been the main reason Lily had even forgiven him in the first place; Lily had been so upset that it must've taken something special to change her mind. It couldn't possibly just have been the silly little gift she'd received. The realization almost made Petunia want to forgive her, too.

When Petunia discovered that Lily was professor Slughorn's favourite student, she nearly felt like strangling her, but only for a moment…because this time, at least she was in a position to compete with her sister, instead of merely being on the outside of that other world, looking in.

The End.

Author's notes: This obviously isn't really a romance as such, but…whoa, Petunia and Snape, fun crack pairing much? XD Still, it just makes some sorta sense to me to have these two talking together. And no, I don't ship Snape and Lily, either. If you want to, you can pretend that Snape and Petunia get together at some point after her venture into the wizarding world; I don't really mind. It's an alternate universe, anyway. XD

The lyrics in the first chapter refer to Petunia's love for her sister, though, if that wasn't already clear. ;)

Heh, I actually had to rewrite some of the dialogue because Snape didn't sound Snapey enough. I guess I'm not used to writing somebody as snide and resentful as Snape. Of course, teenage Snape isn't supposed to be as bad as adult Snape, but Petunia doesn't exactly bring out the best in him like Lily does, I dunno.

Both Petunia and Snape were very difficult to write as teenagers, really. :P

Potions: I assumed even Snape would be a bit chattier when it came to his "area of expertise" and wouldn't be able to stop his curiosity when he saw an ordinary Muggle making Potions. Considering Lily's talent for Potions as well, I figured it wasn't that far-fetched for her to be interested enough to ask around for more complicated stuff.

I also thought that Snape doing something like this for her sister, a Muggle that he dislikes, might make Lily forgive him, especially since what he did remedies some of Petunia's issues with her sister's magical abilities. I'll leave it up to you to decide whether it was done purely to earn Lily's forgiveness or if Snape had also developed a grudging respect for Petunia's unexpected talents and wanted to give them a chance to be honed under the care of Dumbles and Sluggy. :P