Spoiler Warning: This story contains major spoilers for the Half-Blood Prince. If you don't want to get 'spoiled', don't read. Standard disclaimer applies.
The Hardest Thing I've Ever Done
Severus Snape sat in the small, desolate bedroom up the stairs in Spinner's End, nursing a glass of whisky. He had already sent Wormtail packing; it would not be safe here much longer. In a few hours, this place would most likely be crawling with Aurors and Unspeakables as they hunted Dumbledore's killer. He gave a harsh laugh.
The glass shattered against the wall, and Snape leaned forward, elbows on his knees, running his fingers through his hair in a gesture of desperation.
When he had heard about 'the plan' it had seemed paramount to find out – for Narcissa to be that upset, it had to be something big. He had pretended to know what she was talking about, pretended so that she would tell him. But Bellatrix – she had gone on and on about his untrustworthiness, about how he always stayed away from any true involvement: a lot of talk and very little action. If she went back and told the Dark Lord, maybe just asking why he would have told Snape, when only Draco's relatives had been informed… well, the game would have been up. And so when Narcissa had proposed the Vow, he had sworn, not knowing what it was he was swearing to.
He had found out - oh yes, he had found out. And then he had, of course, told Dumbledore…
"It is all rather simple, Severus. When the time comes, you will follow through with the Vow." Dumbledore peered at him over the top of his half-moon glasses, not at all perturbed.
"Kill you?" The Potions master looked at him with disbelieving eyes. "You are needed…"
"When the time comes, Severus, you must do as I wish." Dumbledore's voice grew stern.
"No." Snape crossed his arms over his chest. "You take too much for granted. I cannot and will not do that. There are limits. I took the Vow; I will pay the consequences."
"Severus, you will follow orders," Dumbledore said sharply. "I am old, my health is failing, and time is running out for me as it is. You, on the other hand, are irreplaceable. I need someone who will be able to protect Harry from within Voldemort's circle. What better way for you to gain Voldemort's trust completely than to kill me? One of us, it appears, must die. Your death would serve no purpose. Mine, on the other hand, will."
Severus closed his eyes for a moment before continuing, trying to hold on to composure, his hands balled at his side. "Did it ever occur to you that maybe I don't want to do this any more? I am sick and tired of this game, Headmaster, of this endless charade. You are asking too much."
"Nevertheless, Severus, you will obey me in this. You agreed to do this – I asked you, not ordered you, to resume your role as a spy. You must follow through now, that is all there is to it. You know you have my trust. You will not disappoint me…"
And so it was that when the time had come, he had entered the room and Dumbledore had looked at him. "Severus…please…"
He had hated that old wizard at that moment, hated him for what he was making him do.
He knew what Dumbledore was asking of him. And logically, there was little choice – Dumbledore looked as if he was dying anyway, the room was full of Death Eaters quite capable of finishing the job, and there stood a boy, his soul, unlike his own, still whole and untorn. Dumbledore would not leave this room alive, no matter what happened. And so he had done as he was ordered, cast the curse, not sure at the moment whom he hated more, himself or the old wizard.
It was the hardest, most painful thing he had ever had to do. To kill Dumbledore, the only one who had ever completely trusted him, who had offered him life, forgiveness, and a second chance. To take on the role of a Death Eater from now on, in full, with everyone in the wizarding world, including all Order members, thinking him a murderer and traitor of the worst kind.
And then Potter had called him a coward. A coward… He smiled mirthlessly, bitterly, blinking away tears. If the boy only knew…
He stood up, and picked up the bag with the few belongings he had wanted to take. Time to go, back to the Dark Lord's side, to finish the job he had started. From now on, he would be completely alone.
The dialogue incorporates the lines Hagrid overheard when Snape and Dumbledore were arguing. I think the reason given for Snape's conversion is way too flimsy (even Lupin remarks on how stupid it would have been to trust Snape on that…). And Dumbledore is not stupid. He would not build decades of deep trust on something like that. It is obvious from the book that there is more to it than that. And I think Snape did as he was told. When the book says that Snape looked as if "he was in as much pain as the yelping, howling dog stuck in the burning house behind them" I think that was exactly true – he had just had to kill maybe the only person he actually cared about, as powerless to escape his circumstances as the dog is to escape the fire. Anyways, that is my theory.