Because Dumbledore asked him to,it is clearly verified in the last book towards its end

Answered by shreya at 7:44 PM on August 10, 2007

i think ...snape is a true slytherin given a choice he would save his own life.draco was like his son.

Answered by Alphaa , an ibibo Citizen, at 4:27 PM on July 20, 2007

snape is a true slytherin given a choice he would save his own life.draco was like his son.snape knew voldemort would kill draco if he failed,so he chose to kill dumbledore in desperation even though he wished to remain on good sid

Answered by sos at 12:53 PM on July 18, 2007

Snape turned against the order because that's what he was meant to do all along.It takes a lot of hatred and nerve to perform the Avada Kedavra on a person like Albus Dumbledore specially when he had been almost like a Father to Severus for so long.Snape was a spy and Voldemort's man through and through.Hogwarts was just a safe haven and in order to continue his spying,he needed to be in the order.This way he can convince Voldemort of his loyalty order. This was finally clear when he answers Bellatrix's questions in the beggining of the 6th book.It is fishy characters like Severus Snape who are wicked and smart enough to play a double role..only because they believe in Voldemort.So when Voldemort comes back,Snape makes the choice to go back to him.

Answered by Anonymous , an ibibo Citizen, at 3:33 PM on July 17, 2007

Snape is not hust a double agent, but a triple agent. . Dumbledore believed Snapes sob stroy, that he wasn't working for the Dark Lord, but in true light, he was. only reason Snape killed Dumble dore because of the unbreakable vow he had to Narcissa. And that he only ran away because, after killing the greatest wizard of their time, that he would suffer in Azkaban, and ran to the Dark Lord for safety and to asure that Draco didn't get hurt. so he turnes against the order

Answered by neha , an ibibo Citizen, at 5:27 PM on July 13, 2007