A think I was thoughting about Half Blood Prince
1. I don't know that I necessarily believe this theory I'm about to expound. It's just an idea I had.
2. I doubt this is a new theory. But please humor me and don't tell me how many other places you've seen it, and how much better they did it.
BIG SPOILERS HERE. If you're one of the small handful of miscreants who've taken as long as I did to read book 6, you really don't want to read this.
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I've been thinking about the end of Half-Blood Prince. Specifically, Dumbledore's death scene. And what I've been thinking goes something like this:
We find Dumbledore on a roof, weakened by the potion and surrounded by Death Eaters. He knows he's done. Voldemort is adamant that Draco do the offing, so if they keep standing around while Dumbledore's alive, eventually the Death Eaters will force Draco to act. But Draco's inability to do the deed has given Dumbledore hope that the little brat can someday be won to the "good" side - but only if Draco doesn't bear the stigma of having killed the "good" side's greatest leader.
And Dumbledore and Snape talked about this, over the preceding months when Draco has continued not to kill Dumbledore. And he said to Snape - the triple agent, or maybe quadruple - "Draco cannot be allowed to kill me. If it ever looks like he's actually going to do it, you have to do it first." And Snape growled, and argued, and refused, but eventually Dumbledore got him to promise - maybe got him to take an Unbreakable Vow, which is how he could make another not so different one to Narcissa.
Viewed from this angle, the scene on the roof looks a lot different. Here is Dumbledore, knowing his death has arrived; here is Snape. But now, Dumbledore's plaintive, "Please, Severus," are entreaties not for Snape to save him, but to kill him before Draco has to.
Like I said, I don't know how much I really believe this theory o'mine. It's just another way of considering the situation. Could be Snape's just a double- or triple-crossing bastard who killed Dumbledore because it served Voldemort's purposes - or his own. But she's a sly one, that J.K. Rowling, and I'm withholding judgment on just about everything until I see some proof - and the 7th book.