It was sunny and the grounds around him were full of laughing people, and even though he felt as distant from them as though he belonged to a different race, it was still very hard to believe as he sat here that his life must include, or end in, murder....
OotP, 855-856
The purpose of this (rather scattered and informal, forgive me) essay is to refute this statement of Harry's from OotP. We all have heard or taken part in the raging post-HBP debate of whether Snape is evil or not, but for this essay I'm going to assume he's still on the side of good, mostly because I can't fathom that he's been ese! all along, but also because I think it fits structurally (which is another essay for another day, and one that I'll write eventually).
So, moving on from there. One thing that hit me as I was watching Goblet of Fire this weekend was the graveyard scene, and by the graveyard scene I mean this bit in particular:
Voldemort was ready. As Harry shouted, "Expelliarmus!" Voldemort cried, "Avada Kedavra!"
GoF, p 663.
Actually seeing it brought home one crucial fact that just didn't hit me as strongly in the book - Harry is psychologically incapable of killing someone. Or, to be more explicit, he was incapable of this as of the fourth book, and in OotP he can't even manage a decent 'crucio' against Bellatrix Lestrange; a more deserving recipient of torture you could not find. So the question now becomes, will Harry gain the psychological capacity to do this in the seventh book? My guess is that he won't be able to. Throughout the books, Harry has come up against foes that make most of the wizarding world metaphorically pee their pants in fear, going so far as to embrace a smear campaign against their heretofore hero rather than accept his word that Voldemort and his Death Eaters are once again a threat. And every single time he comes up against some bad guy, he uses the most basic spells - expelliarmus, the jelly-legs jinx, stupefy, etc - and, even more amazingly, he gets away. This is an important point, and I'll come back to it a little later. For now, it's only important to note that he only reacts to danger. He doesn't actively seek to destroy the Death Eaters or Voldemort, though the world would undoubtedly be a better place without them. He is focused on getting away, and he uses schoolboy jinxes and spells to do it.
Another reason I believe that Harry won't be able to murder Voldemort, though a much more subjective one that deals with the author's intentions, is based on the description of Horcruxes in HBP. Slughorn tells the young Tom Riddle that murder/Horcruxes are the ultimate evil, and that murder rips the soul (forgive my lack of direct quotes for this part, as I've loaned my copy of HBP to a friend). The ultimate evil. Somehow, in a series that is obsessed with how choices define people, I can't see JKR allowing her hero to succumb to the "ultimate evil," even if it is to save the wizarding world. This would taint him, and in a way make him no better than Voldemort. Harry senses this in OotP when he's on those sunny grounds, surrounded by laughing people. If he murders Voldemort, than he is becoming that which he hates. I don't think this would be an acceptable option to Ms. Rowling, nor to Harry.
So if Harry can't kill Voldemort, then somebody must do it. The question then is, who? I would argue that it has to be Snape. There is no one else in the Order that, canonically, has killed (please correct me if I'm wrong on this issue - did another member kill in the battle at the end of HBP?). More convincing is why Snape killed Dumbledore, if we take for granted that Snape is still on the Order's side. Snape killed in order to save Draco's soul, so that Draco didn't have to rip his own soul (if he ever could have murdered Dumbledore in the first place, which is iffy) or die when he failed his mission. Snape has one tear already in his soul from this murder, and he probably has more, based on what we're told of what the Death Eaters get up to. What's one more little rip, one little murder to save Harry Potter's immortal soul? Harry can use his first-year spells to destroy the horcruxes to make way for the final battle, during which Snape can use the big-boy spells of Avada Kedavra and Crucio, and Harry will still get away, just like every other skirmish in which he's been involved.
I think in the seventh book that Harry will destroy the horcruxes, but that someone else, Snape, will have to carry out the final act of killing Voldemort himself. Personally, I think we're going to go through the entire seventh book being told Snape's a bad guy while Harry deals with the horcruxes, and in the final showdown, trio versus the Death Eaters and Voldemort, Snape will reveal himself to have been on the good side all along by doing the one thing that will prove his goodness - killing Voldemort. It has a nice bit of irony to it, doesn't it?; Snape kills the major players of both sides in order to prove his loyalty to one of them.