Stop the presses!!!!
I was listening to the audio book and this paragraph struck me. I just figured something out I think. page 55.
"The magic I evoked fifteen years ago means that Harry has powerful protection while he can still call this house "home" However miserable he has been here, however unwelcome, however badly treated, you have at least, grudgingly, allowed him houseroom. This magic will cease to operate hte moment that Harry turns seventeen: in other words, at the moment he becomes a man. I ask only this: that you allow Harry to return, once more, to this house, before his 17th birthday, which will ensure that the protection continues until that time."
It seems that Dumbledore's goal was to bring a wholesome, healthy Harry Potter to manhood in the face of incredible odds.
My theory is that the spell that Dumbledore cast was sealed with his life, as the protection spell that saved Harry from the Avarda Cadabre spell was bolstered by his mother's sacrifice. Perhaps in an act of contrition for exposing James and Lilly to Voldemorts wrath, Snape had made an unbreakable spell to Kill Dumbledore at the proper time. It would take someone with a dark background, someone bearing the dark mark, in order to cast an unforgivable spell, and Snape had to seal Dumbledore's sacrifice. This would explain the anger Snape showed when he killed dumbledore as hatred is nescesary to cast the death spell. I gotta think Snape is still on the right side, but had to perform a dark act to the benefit at all.
