DrHouseHasAspergers wrote:
Dumbledore's death was planned between him and Snape. Dumbledore was going to die anyway because of the curse from Marvolo's-ring-turned-horcrux-by-Riddle. And if Snape didn't do it, Draco would have been killed because Draco was supposed to kill Dumbledore, but he couldn't because he was too scared.
Or even worse; Snape would also have died if he had not killed Dumbledore, because of the unbreakable vow to protect Draco...
Dumbledore had to be killed by Snape, else Draco would have been killed for not fullfilling his task, Snape for breaking his vow and in the end Dumbledore would die anyway because of the ring...