(Part of a many part series of posts called "Whose Side Is Snape On?")
Just before I drifted off to sleep last night, I had a thought about this.
It's quite probable (although definitely not a certainty) that Snape had a pretty good idea of what Dumbledore was doing horcrux-wise. He helped Dumbledore after Dumbledore removed the horcrux from Marvolo Gaunt's ring; he was the first person Dumbledore asked for after drinking the awful potion.
And we know Snape isn't stupid.
If Snape is evil, he would have (or shortly will) go straight to Voldemort with that information. If that happened Voldemort could easily retrieve his own horcruxes or stake out the places where they are, and prevent Harry from ever even having a chance to kill him. I don't know about you, but I never saw the books ending that way.
Yes, I think it's possible that Harry might die in the end, but don't think he'll die and Voldemort will win. I think if Harry dies Voldemort must also die. If Harry doesn't get all the horcruxes before facing Voldemort that can't happen. In order for Harry to get a fair shot at Voldemort, Snape, at the minimum, must be working for himself alone. And given the emotions he displayed before and after Dumbledore's death it's more probable that he's still on Dumbledore's side.