In the last few days, I've exchanged emails about Harry Potter and Philosophy with Tim LeBon, author of Wise Therapy. I thought I would post some of my thoughts that I shared with Tim on the upcoming release of Harry Potter and the Deathly Hollows. [Edit: Read Tim LeBon's blog entries that incorporate my comments and the comments of Ed Kern and Tim Morris.]
Regarding Snape, obviously, we won't know for sure until we see the last book, but I think ultimately Snape is redeemed. As you said, if Snape turns out to be truly a death eater, then Dumbledore has been made quite the fool. Snape has been the red herring in each of the books, and I think he's still the red herring. Then again, Dumbledore has admitted to making mistakes (at the end of Bk V where he takes some of the responsibility for Sirius death and for placing Harry into more trouble than Dumbledore expected). Moreover, maybe the biggest red herring of them all is that Snape really is evil. As the saying goes, the best hiding place is in plain sight. This scenario is, I think, unlikely. It would end the series on such a sour and malevolent note.
Which leads to the question of whether Harry will die. Harry's death would also end the series with a malevolent feel. Moreover, I think it would be in sharp contrast to almost everything in the series. This series is fundamentally a story about moral development; it is a story about Harry becoming a responsible and mature adult. His death is not the logic progression here. The logical end is his independence and the flowering of his power. Harry's development towards independence has been a central theme: from escaping the Dursleys to losing Sirius and now Dumbledore. I think we will see Harry take full control of himself and his powers and take his place in the adult world.
JKR has left it so that she can really do anything. There are 700 some odd pages left and a lot can be revealed that can up-end the best predications. From a fan point of view as well as philosophical/artistic point of view, I don't want to see Harry die. I say personally because I like Harry and I like happier endings. I say philosophically because I don't think killing the hero of the story is consistent with the kind of story of moral development and growth that JKR has been telling. I say artistically because the ultimate point of art is to uplift our souls, provide us with strength, and give us insight into our selves and to our lives. I don't see how Harry's death would serve those ends.
But if Harry must die, I don't want him to die in some grand sacrificial manner that casts him as some kind of Christ-like figure. Such an ending would be personally unsatisfying, but also against the grain of the whole series. The imagery and symbols have largely been drawn from classical and pre-Christian culture and so pasting a specifically Christian symbol on to it at the end would be incongruous.
If I had to guess, I'd say Hagrid is going to get killed. I suspect possibly a Weasley family member(Percy?) and maybe even a Dursley family member (Petunia?) But these are really just guesses, hunches.
Ultimately, however JKR close the series, the path she takes us on to that end will be more important than how it ends. Whether Harry lives or dies, whether Snape is evil or not, what will matter is if these last 700 pages tell the story in the way that makes it so when the end comes it is what we will need to see.