I`ve answered "According to the Prophesy, Harry has to be the one to kill him",
but it is possible "You-know-who is the real Hero of these books", because
if Voldemort would be dead, there wouldn`t be the one, who Harry is fighting, and,
at last, Harry woudn`t be able to revenge for his parents.
And there is a possibility that Dumbledore couldn`t kill Voldemort. That`s also possible!