Conditions for an "unforgivable curse" to work an alternative interpretation of D's death:
Has no one noticed that Rowling, in the tradition of British mystery writing, always gives us the clues we need to put it together? Sometimes you can't do the math till after the revelation, but it's all there if you go back and look for the hints. So, regarding the forbidden curses we are told at least 3 times over the course of the series that in order to perform an "unforgivable curse" you have to 1) have the power (which Snape does) and 2) really mean it (which Snape may not.) We are given these conditions by the faux Moody in Book 4, and again by Bellatrix in Book 5 (during the fight scene when Harry tries to use "Crucio" on her in the entrance hall to the Ministry of Magic. It works, but only in a reduced degree, because he is not sufficiently hateful.
Snape even alludes to these conditions in the 6th book after he has "killed" Dubledore and is fleeing from the castle but turns to confront Harry who is pursuing with intent to kill or at least torture Snape. Harry starts the Crucio curse but Snape deflects it and says "No Unforgivable curses from you Potter!...You haven't got the nerve or the ability--" (US ed. p 602.) So I have to ask, if Snape didn't have sufficient intention or didn't "have the nerve" could he have failed? Or, more likely, could he have been faking the look of hate and malice in order to convince the Death Eaters that he had killed Dumbledore? Or, perhaps the look of hate was genuine, but not directed at Dumbledore. (More on this below.)
After all, we know that Snape doesn't need to vocalize in order to perform a curse, so might he have been performing the "Wingardium Leviosa" charm silently in order to levitate Dumbledore off the tower while appearing to be killing him. The Avada Kedavra curse need not blow anyone up into the air from what I can determine. This would explain several anomalies. Specifically it explains why:
1. Dumble dore let a child disarm him 2. Why Dumbledore appeared to be helpless, even though we know that wands are only conduits for the wizards power. (Remember, Harry has performed several acts of self-defence without one before he even cam to Hogwarts.--See book one.--and again he blew up his aunt in book 3.) 2. Why Snape, who supposedly despises Harry, not only protected him from another Death Eater, but admonished him to learn how to do silent curses and "close his mind" through occlumency even after Snape had "killed' Dumbledore and had no reason to fear the consequences of killing Harry. (US ed. p 603.) 3. The Avada Cadavra curse is not necessarily specific to people. "According to J.K. Rowling, the phrase Avada kedavra 'is an ancient spell in Aramaic, and it is the original of abracadabra, which means 'let the thing be destroyed.' Originally, it was used to cure illness and the 'thing' was the illness, but I decided to make it the 'thing' as in the person standing in front of me." The word "kedavra," probably coincidentally, is similar to the word "cadaver," meaning 'corpse.'" (quoted from Answer.com http://www.answers.com/topic/list-of-spells-in-harry-potter
.) So it may be that the target was the locket in Dumbledore's pocket. If so then Dubledore was levitated off the tower and the part of Voldemort in the locket was destroyed. But that would make the body of Dumbledore a mere Doppleganger and the locket with the note a mere plant to deceive the Death Eaters and everyone else into thinking that Dumbledore is dead and the threat to the Horcruxes is gone. 4. #3 would then explain why Snape was so paniced when Harry started to say "leviosa" on page 64...could the pronouncement of "Leviosa" have caused Dumbledore's suspended body to fall. He might have been unconscious or injured due to the proximity of the killing curse. 5. It explains why Fawkes, the phoenix (who is unremittingly loyal not only to Dumbledore, but to anyone that is also loyal to Dumbledore)did not show up to save his master. (Remember, he took a curse from Voldemort in Book 5 to save Dumbledore.) And we know htat he is the only creature capable of Apparating within the grounds of Hogwarts. 6. And finally, it explains why Hagrid tells Harry "Snape kill Dumbledore--don' be stupid, Harry....He couldn have. (US ed. p607)" As we should know by now, Hagrid is wiser than he appears and speaks the truth when all others are deceived. Sometimes, he's wrong, but not about Dumbledore. Rowling uses him as her oracle on a regular basis. I think we should heed his words now.
If I am right, the entire 6th book is a set up for a grand revelation in book 7. We are coming to the show down and Dumbledore will show up at thecritical moment as he did in book 5. Snape will be revealed as a loyal member of the order of the Phoenix who has gone into deep cover (by faking D's death) in order to penetrate the Death Eaters at the highest level. Even if he really killed Dumbledore (which explains the "please" from Dumbledore to Snape) Fawkes will play a role in his resurection. (Remeber the apotheosis at D's tomb, wherein Fawkes did his fiery exit over the body?) Dumbledore has often said there are worse things than death...perhaps because he has fould a way to defeat it.