Disclaimer: I don't own Harry Potter.
"No. Absolutely not. I refuse."
He crossed his arms and did his best to stare down his nose at he had once done with his students. Staring down your nose is awfully hard to do when the painting you're talking to is hanging higher than your own, though.
"Come on, Snape. We all know you should have done it ages ago. I'm sure you'll feel better for it!"
The gleam in his eyes spoke of malevolent amusement rather than any sort of concern for Snape's well-being.
"Surely, Nigellus, you have better things to do than tell me how I feel."
He cursed himself for that utterly weak retort. With so little else to do, he really ought to improve his rhetorical skills. His opponent seemed to agree and smiled at him in a sneering way that might suggest he was going for the kill this time.
"And what would those better things be, Snape? Surely you've discovered how very uninteresting life as a painting is. The world seems to have shrunk to include only this office, and thus everything going on in here is of vital importance if you want to keep your brain from shrinking, shrivelling and dying from boredom!"
A very good point indeed. The problem was that this held true for all of the other thirty-seven paintings in the room as well, which resulted in very little privacy.
"That... brat hardly does anything good for anybody's intellectual health! And what I say and don't say to him is nobody's concern but my own."
Nigellus broke down in evil-sounding snickers at that.
"Dream on, Snape. And while you're at it, you can also dream on about not being scared of him. You can't even say his name! Has Harry Potter really gotten scarier than the Dark Lord?"
Before he knew Nigellus as more than just one of the thirty-eight paintings in the Headmaster's office, Snape had honestly believed himself to be an angry, bitter man whose only purpose in everyday life was to make his surroundings as miserable as possible. Now he knew that he didn't hold as much as a match-stick to the acrid poison that inhabited portrait number thirty-three. Said spiteful existence ploughed on without needing to breath (being a portrait had to have some advantages):
"I do concede, of course, that he's horribly boring, but you're well and truly naïve if you expect anything else from a Gryffindor. Having that sort of moral codex badly limits creativity. I don't get why you're so afraid to test it. If I weren't so disgustingly sure that I'd win, I'd bet you any time that he'll look sadly at you, making you feel all guilty and horrible, smile, and say that he understands and of course he'll forgive the unforgivable. That's why I despise Gryffindors so much: General boredom is one thing, but when predictability becomes so predictable that you make predictions about it, you just feel like getting in the way of a Reducto curse."
The problem, Snape thought, might be that Nigellus had been here too long. He'd seen too much, gotten used to too much, and the only things that seemed to bring him any pleasure was insulting anything that moved, and being surprised. And surprises were admittedly hard to come by these days. That just didn't explain why all the other paintings seemed to be overall content with existence. Perhaps they knew of some technique to hibernate when nothing of interest was going on. Like they pretended to do now. Or maybe didn't pretend. Fights between Nigellus and himself had become far too frequent to be interesting. He was pretty sure that Albus was listening, though. Nigellus was still spitting acid:
"They are plain disgusting, really. They should be wiped..."
That's when the door opened to admit Headmistress McGonagall. There was no denying that she was getting old, but today there seemed to be a certain spring in her steps.
"He's here." She announced. "And he brought miss Weasl... I mean, his wife, and their youngest son. Albus, they are so happy that they seem to glow!"
On the wall behind her, Nigellus made a disgusted face. "And you are getting distastefully sentimental. But what would you expect from a Gryffindor..." he muttered, which she chose not to hear.
Snape observed Minerva and Albus gush about Potter and his family; happy that Nigellus' attention was elsewhere, but feeling a nervous break-down nearing. As maliciously as he had said it, Nigellus had been absolutely spot on. He wanted, no needed, to tell Potter that he was sorry. Not ask for forgiveness, because he knew perfectly well that what he'd done was unforgivable, but for some reason he couldn't stand the thought that Potter might be thinking that he didn't have regrets. He was really getting soft. Honestly, you'd think he was some girly watercolour painting and not a stately oil piece.
Potter had visited the office several times in the last few years. When he'd shown up to tell McGonagall in person that he was getting married, Snape had pretended to sleep. When he'd shown up with their first-born, he'd hid just outside his frame. He'd felt the oil almost boil when he caught a glimpse of unruly black hair and heard the name James Sirius Potter. Really, could it possibly get any worse? Here was the unshakable proof that Harry would never forgive him. Really, that name had probably been chosen just to torment him.
"Severus, are you planning on staying this time?" asked Albus in an undertone when McGonagall was busy telling a house elf to get tea and biscuits. He'd been rather annoying about this whole story. When all the other paintings (even Nigellus) had been rather too busy telling him to do the right thing and talk to Harry, Albus had just smiled kindly and told them to leave Severus alone. This, he'd said, was something Severus had to work out on his own. Severus knew the old headmaster to never not try to interfere, and the fact that he didn't seem to be doing it now was quite unsettling. Unless he thought it would be more efficient to let Severus have the impression that nobody was pressuring him.
A knock was heard and McGonagall all but flew to the door. It opened to reveal three fourths of the Potter family. They'd left the oldest son at home, it seemed. All the paintings fell silent and all eyes seemed to magnetically turn towards Severus. Even McGonagall was looking at him. Potter caught on (surprisingly quickly for him) and apparently understood that something important was going on, because he took a few steps towards Severus' painting with his newborn son in his arms. "Professor Snape." he said with a nod in greeting.
Severus swallowed one last time and send a few prayers to whomever would be stupid enough to listen. "Harry Potter." He was rather proud that his voice was calm and even a little cool. "I need to speak with you." It sounded dignified and as a request more than the wild plea it actually was. Harry nodded again to show he was listening. "I have done things in my life that I... regret. I haven't... treated you... well. Or fairly. And you might have realised that it wasn't because you're your father's son as much as because you're your mother's and your father's. But that was hardly your fault, and it also isn't your fault that your looks make that impossible to forget. I treated you like a fowl enemy when you were just a child, I punished you for crimes you didn't even know had been committed, and I took pleasure in violating your privacy and humiliating you. I'm sorry."
During this speech, Harry had been looking at him, but not, as Nigellus had thought, in a sad, understanding way. His gaze was open but not aggressive and he waited calmly until Severus had finished. Then he spoke: "I can't forgive you." He held up a hand when Severus wanted to say that he hadn't asked him to. On the wall behind Harry, Nigellus was... well, gaping. "I can't forgive you" repeated Harry. "Not because I don't want to, but because I won't... can't, actually... judge you. For several years I hated your memory, even if I did it in secret. Intellectually, I knew that what you did was brave and necessary, but I couldn't excuse what you'd done to me." He paused briefly and tightened his hold on his son a bit. "I have a family now." He said. "I have wife and children and I think... I think that I understand." He seemed to be struggling with the words, but that didn't matter, because he looked Severus in the eye and seemed to be willing him to realise what he meant. And what Severus saw was that he did understand. Was it loving a red-haired woman that made that possible?
Harry swallowed and continued: "I can't forgive you, but I can't condemn you either. I think the only person in this world who has any right to judge is yourself. What I can give you is my respect. And good wishes. Even if I can't forgive, I'm starting to forget. I want peace."
The office was dead quiet. People and paintings were all holding their breaths. Except the tiny boy in Harry's arms. He chose that moment to wake up and make a wondering sound to his father. Then he turned his big green eyes towards Severus and fell still.
"Oh, yes," said Harry "And I wanted you to meet my son. His name is Albus." In the frame next to Severus, Albus started sniffling. He, too, had forgiveness to ask, but it seemed it had been granted already. "Albus Severus Potter." added Harry without looking away from Severus. The paintings broke out in chatter and McGonagall seemed stunned but pleased. Severus could only stare. At the little boy whose name meant so much, and at his father who refused to look away. Severus wanted to say something, but had no idea what. Harry nodded again, this time in good-bye and perhaps with some sort of "see you" implicated, and the whole family and McGonagall left.
The paintings were still chattering loudly, but nobody was paying attention to Severus, for which he was grateful. He needed some time to digest this. Then Nigellus' eye caught his. The old Slytherin just smiled crookily and not at all pleasantly and lifted an eyebrow in a gesture that quite clearly said: "Told you so."