I think the biggest key in this scene, though, is Dumbledore's character. Can you imagine the Albus Dumbledore we've known all this while begging for his life? Dumbledore does not fear death--we've had that pointed out over and over. Ergo, if he is not begging for his life (which I think he would not do), then he must be begging Severus for something else.

I think--given that everything we've ever seen suggests that Snape tells Dumbledore everything--that Snape would have told Dumbledore about the vow.

"Why didn't you [Dumbledore] stop me then?" Malfoy demanded.

"I tried, Draco. Professor Snape has been keeping watch over you on my orders--"

"He hasn't been doing your orders, he promised my mother--"

"Of course that is what he would tell you, Draco, but--"

It doesn't sound to me as if he did tell Dumbledore about the Vow, because Dumbledore seemed to think Snape was spinning stories for Draco. Assuming that's true, and Snape didn't tell Dumbledore, why? Wouldn't that be his first priority as a spy? To tell about a deal that involved him having to aid Draco in killing Dumbledore or do it himself... or die?

This passage does beg the question of whether Draco knew about the Vow. And I don't think it's clear whether he did or didn't. Would Narcissa have told her son that if he failed, Snape would come to the rescue and complete the task? And I honestly don't know the answer to that question.

"Listen to me," said Snape, his voice so low now that Harry had to push his ear very hard against the keyhole to hear. "I am trying to help you. I swore to your mother I would protect you. I made the Unbreakable Vow, Draco--"

"Looks like you'll have to break it, then, because I don't need your protection! ..."

The Unbreakable Vow, pg 323, scholastic

And now the hard part. Snape.

I'm not convinced he's evil. I'm almost convinced, I'll admit. But there's still a nagging part of me that says there has to be more to him... There were just some seriously fishy things...

If JKR wanted him completely evil, I don't think she would have foreshadowed it so freaking obviously in just the second chapter of the book (I think it was the second?) with the Unbreakable Vow. He hesitated to accepting the vow, which may have been just out of an understandable fear... Dumbledore is a formidable opponent, and most Death Eaters would be loathe to attempt finishing him even if they found him sleeping on a bench somewhere in the open, completely vulnerable.

I think there's going to be some serious plot concerning the tie between Snape and the Malfoys, particularly Draco.

I'm not sure. Maybe Snape's got too much of that Slytherin arrogance, and he thinks Dumbledore was a necessary sacrifice so he can keep his cover as a Death Eater to work toward ridding the world of Voldemort permanently. The one thing that gives me the most pause in thinking he may not be as evil as he's made out to be is his continued absolute hatred of Harry. When Harry was chasing him... he uncharacteristically lost his cool with Harry completely... Shouting about being the Half-Blood Prince, how weak Harry is, using his own curses against him, like his father did... I'm... entirely not sure. On the one hand, maybe he's pointing out very not-subtly Harry's flaws. Harry's currently no match for Voldemort, I think, even given having all the Horcruxes destroyed. But... Maybe Snape really was just finally telling Harry exactly how he feels -- why not, after all? No hope of coming back to Hogwarts or anything, everyone's to think he's evil, whether he actually is or not... Hm.

And when he raced up into the tower. He's quick, so he may have taken in that Draco wasn't going to work up the nerve to kill Dumbledore and that they needed to get the hell out of there as fast as possible... but I think it's still fishy. Draco wasn't precisely in mortal danger. He'd probably be less likely to get killed if Snape goaded him into killing Dumbledore. Hell, he's so good at nonverbal curses, he could have freaking momentarily Imperiused Draco and had him kill Dumbledore so Voldemort would be pleased. I don't think Snape's in it to steal Draco's glory, the way he was complaining initially. So I don't know what was going on.

Dumbledore of course knew he could die. He was teaching Harry all he could to go on with to fight Voldemort should Dumbledore not be able to help him. (That sentences makes sense if you think about it hard enough XD) But maybe... maybe he guessed that he probably would die... maybe he talked to Severus about it... maybe it was part of some form of last-ditch plan...

We never did hear what had Dumbles so thoroughly convinced about Snape. I don't think it was simply being sorry over James and Lily's death. He knows Snape isn't stupid, and he knows Snape hated James with a passion -- if nothing else, his continuing grudge against Harry stood as proof he hadn't forgiven James. No, I think there was something else that had Dumbledore convinced.

Snape is their only Death Eater spy; Draco is their only Death Eater spy potential. Dumbledore wanted to save Draco, albeit not necessarily as another spy -- but that's Dumbles, and we shouldn't expect less from him. But still. Snape is awfully deadset on protecting Draco, I think more than a simple Unbreakable Vow could account for. Maybe Dumbledore found Snape's position as a spy among the Death Eaters to be more important than his own life, should it come down to it. Even if none of the Order believes Snape is on their side anymore, whether they realize it or not, they may have someone on the inside of enemy ranks working for them...

So I still think there's far more to Snape than meets the eye, or perhaps the casual read.

Anyway, simply having to find the remaining Horcruxes, destroy them, and face down Voldemort hardly seems like JKR's type of plot, especially not to finish up a series like Harry Potter. She'll undoubtedly have even more twists up her sleeve in the final book than any of us humble readers could imagine. I think Snape will be a part of it, but I can't hope to guess what sort of things JKR has planned for poor Harry and the others.

...

And also, I wish Harry would have talked to Dumbles' portrait newly on the wall of the headmaster's office. I hate how he doesn't think of little things like that. I think portrait-Dumbles might have had some serious advice for him. Might even have had something to say about Snape. I don't know, since there's no telling how heavy an imprint of the wizard those portraits actually contain... but... I wish Harry'd talked to him. Jeezus.

I don't know about that. He seems to be very able to meet those eyes with pure hatred and I don't recall Snape mentioning the eyes or in anyway indicating that he noticed them, but perhaps I missed something.

I thought it was interesting that JKR had Snape look at Dumbledore with hatred right before he killed him, but perhaps that was him simply preparing himself for the Killing Curse.