Disclaimer: If I owned it, I wouldn't be writing fanfiction about it.
Spoilers for HBP! (Really only for the second chapter, but hinting at things that show up throughout the book, and especially at something that happens near the end…)
A/N: Well, my cousin gets credit for this idea; after I'd already finished HBP, she was still reading it, and told me a theory that Snape was never really good or bad, that he was just playing both sides waiting for one to be a winner. I was reminded strongly of Phineas Nigellus's quote, in OotP, about how a Slytherin will always choose to save his own skin… So I decided that I like that theory, but considering the end of the book, Snape has to have made a decision at some point to be on one side or the other. Once my cousin finished the book, she told me that this was when she thought Snape had made that final choice, and finally crossed the line… Very, very different from what I usually post, but oh well.
Crossing the Line
He is standing on the line, and he knows it: the line between conscience and common sense, between heart and head, between good and evil. It's never been an easy decision for him; he's taken to hating those who are so sure of themselves. Certainty, he knows, is deadly. All that has kept him alive these sixteen years has been his indecisiveness, his refusal to pick a side.
At one point, there had been no question; at one point he was Voldemort's man, through and through. The Dark Arts called to the sickly, bullied child he had been, given him power others could not turn against him. For once, he was accepted—for once, his talents were noticed, his intelligence praised. He found solace within a secret, elite society led by one of the most powerful wizards of the age.
But he knows that time of comfort to be merely the foolish daydreams of a child who has yet to grow up. He is an adult, now; he knows that life is never that easy. Now, he understands that all life is spent torn between extremes, and that those who take the middle road live the longest. It took the disappearance—he can no longer say "death"—of his old master, the most powerful Dark wizard of the age, to show him that.
He knows patience, now, another things he could not comprehend before; he has known waiting and wishing. The first, in some cases, might be noble; the latter, he knows now, is a sign of weakness.
But he did not know it then, when his master died, and he was suddenly stuck in a position he never truly wanted, with no way to escape. Potions, he thinks with at shudder, why potions? Surely, though he was a genius at the subject, it was not the one which he was best credentialed to teach. Of course, Dumbledore disagreed; Dumbledore, to whom he was forced to turn in order to save his own skin, that horrible day he found out Voldemort was dead.
He recoils at the unpleasant memory, but also at the small part of his soul which appreciates and respects the man who took him in knowing he'd been a Death Eater, knowing he's delivered crucial information to Voldemort, knowing all sorts of things that he prays no one else will ever know. As Voldemort's man, he is valued; but as Dumbledore's, he is trusted. His tortured soul, forced to choose between the two, finds such a choice impossible. What started as a safe bet—playing both sides so that no matter who won, he'd come out the winner—had become more dangerous than he expected. Now, both sides require of him too much.
Both sides require of him his life.
He is in over his head, but his hands are chained; he can no longer treat water, and is sinking, fast. Two hands reach to help him, two very different hands, and he cannot decide which to take, does not know which one will let him live, is not even sure any more if life is a possibility.
But the darkness clears, and he is left knowing that there was only ever one hand he could reach for…the hand of one who, like himself, sees the distinct advantage to saving one's own skin.
"Certainly, Narcissa," he says calmly, holding out his hand. "I shall make the Unbreakable Vow…"