I've been trying to find an answer for this at the books, and I don't see it clear...
He really needn't to do it, he could have just killed her, so why did he give a chance?
That's not "normal" in Voldemort (I mean, if we consider "normal" killing people, horcruxes... what he does is never normal).
But he told her something like this "Give me the baby and you could live". Why should she remain alive? Why was she special/different?
I read before, somewhere here, some theories about Snape being in love with her, and I agree with that, but could he maybe told Voldemort not to kill her?

Do you know anything more about this?