Maybe Snape has already saved Harry once, but if the prophecy was about him, he would have no choice, would he?! And so far it's just Dumbledore's interpretation that the prophecy is about Harry and even Dumbledore makes mistakes...And remember, Rowling is very tricky, she always lets us believe the contrary of what's true. I think she wants us to forget about the words of the original prophecy, she wants us to remember only Dumbledore's words! And there are some little sweet differences between Dumbledore's and the original words. Maybe it's not Snape, but I think it will someone else who has to kill Voldemort.