Then again, I'm sticking pretty firmly to the 'Dumbledore Asked Snape to Kill Him' - theory, because any number of things don't make sense in this book, if Snape's really switched sides.
I feel like making a list :
- Snape's hand twitches when he has to agree to the final condition of the Unbreakable Vow, which we now know was to agree to finish off Dumbledore, should Draco fail to do so. Now, Dumbledore gave Harry very specific instructions when taking him along when going to get the Horcrux (Harry was even supposed to obey orders like 'run away' or 'leave Dumbledore and rescue yourself'.)
It seems very likely that Snape had equally, if not more, specific intructions for his work as a spy. If he had not agreed to make the Vow, we know for sure that Bellatrix would have forever gone on distrusting him - and possibly Narcissa as well.
Maybe to agree to this kind of thing was perfectly within his instructions. When he heard the last condition of the Vow, he didn't want to agree to it - hence the twitching of his hand - but he finally stuck to his instructions.
- Dumbledore gave Snape the DADA job. He knew he would lose Snape that year by doing so. Logically enough, that's why denied the post to Snape in all the years before. It seems plausible that Dumbledore would have decided to give Snape the job he always wanted after he heard from him about the Vow. It's almost like..." I grant you your last wish, now that I know we have to part"... Boohooohoo! Sad!
- Dumbledore repeatedly says that he "trusts Severus Snape completely" (he even gets angry when being questioned too much about whether that trust is warranted). And despite everything that JK says now about Dumbledore being isolated and really, really wanting to trust SOMEONE, it just does not seem likely for him to judge a character so completely wrong. He IS the greatest wizard of all time, & from how things look in the book, he was COMPLETELY deceived by Snape at all times, while Harry was COMPLETELY right with his suspicions. Fishy... *g*
- Another thing, although this one's far from conclusive in any way, about Dumbledore saying he trusts Snape is that in the end Harry says to Rufus Scrimgeour that he's "Dumledore's man through and through" and that Dumbledore didn't wish to share his knowledge with the Ministry and that Dumbledore's word STILL COUNTED. Dumbledore's words count even after his death.
If might be that this includes everything he said about Severus as well.
- According to Hagrid, Snape says, when arguing with Dumbledore, "that perhaps Dumbledore took too much for granted". It MIGHT mean that he didn't want to kill Dumbledore and that he thought Dumbledore took too much for granted in assuming that Snape would follow his orders that far.
- Dumbledore repeatedly tells Harry that Harry's blood (& his life) is worth more than his own. Clearly, this implies that he'd die to save Harry.
Why wouldn't he also die to save Draco and Severus, then? Who are also both younger and fitter, "worth more", than him.
- Dumbledore tells Harry that he only survived the curse that withered his arm because of "Snape's timely help". Why would Snape's help be so timely if he could've done away with Dumbledore simply by coming a little later?
- When Harry asks Dumbledore why he trusts Snape, Dumbledore is contemplating something, but in the end he doesn't answer. It seems as if he was contemplating whether or not to tell Harry, but decided against revealing Snape's secrets.
- Dumbledore says that he thinks it was the "greatest regret of Snape's life" to have told Voldemort about the prophecy that led James & Lily to their deaths. Dumbledore KNEW, however, that Snape hated James. There must be a missing piece here!
- Upon coming back to Hogwarts on the night of his death, Dumbledore tells Harry to go to Snape for help. ONLY Snape. However, when Draco appears, he IMMOBILIZES Harry suddenly. What possible reason could he have for making Harry unable to move, to get to Snape, other than that he might not have wanted for Snape to come anymore then. Maybe he wanted for Draco to kill him, so as to spare Snape the pain of having killed his mentor.
- Dumbledore never says, "Please, Severus, HELP ME" or anything, when Snape arrives. He only says, "Severus, please". It also seems unlikely that he would even WANT for his spy to reveal himself IN FRONT OF a group of Death Eaters just to help him. Especially if we keep Dumbledore's earlier remarks about how Harry was worth more than himself in mind, it seems unlikely that he'd even want for Snape to try to rescue him instead of himself in that situation. It is, after all, highly implausible that Snape would have been able to fight all those Death Eaters before they killed both him AND Dumbledore.
- Dumbledore is not afraid of death. Why would he plead for his life?
- Dumbledore and Snape stare at each other before Snape casts Avada Kedavra. They are both capable of Legilimency.
- Snape has a look of loathing on his face when he kills Dumbledore. Maybe that loathing wasn't directed at Dumbledore, but at himself or the Death Eaters.
- In his flight, Snape could have easily hurt Harry or taken him with him to Voldemort. He does neither of these things. ALL he does is block Harry's spells. What possible logic could be behind THIS, if he is indeed Voldemort's servant?
- Snape freaks out the MOST in the whole book when Harry calls him a coward. He will deal with being called a killer and a traitor, but he cannot stand THAT. Maybe that was because he had just killed his mentor and the only person who trusted him on that mentor's orders? An act which effectively leaves him completely on his own, in the middle of two opposing sides in a war...
- Snape is practically a genius. We know this now. He invented spells while he was still at school. Why would he feel the need to show complete deference to a master to whom he is no more than a servant, nor will he ever be?