Before you click on the cut below, I don't wish to sound all arrogant-like, but I did totally call one of the biggest shockers of Book 6 two days before the novel came out. So even though this is pure speculation on my part, and not based on any inside knowledge or even rumor about the contents of Harry Potter and the Deathly Hallows, you might want to steer clear if you're hoping to be surprised.
I wanted to write this up in a more sophisticated, essay-like form, with references and whatnot to back up my ideas, but I don't have time for that right now. So I am afraid you will have to put up with a condensed version of a very informal chat I had with
cesario instead, for which I do apologize.
You know how we've thought until now that Snape was protecting Harry in PS/SS (and afterward) because of his life-debt to James?
And we've been wondering about that gleam in Dumbledore's eye?
And also why Dumbledore trusts Snape absolutely?
Plus a lot of other things including the prophecy that "neither can live while the other survives"...?
Well, I believe that we were introduced to the concept of the Unbreakable Vow in Book 6 for a very important reason. Because after the young Snape realized that he was inadvertently responsible for Lily and James's deaths (as I think he was, by telling V. the Prophecy), he swore an Unbreakable Vow to Dumbledore to protect the young Harry with his life. After all, they both knew that like it or not, Harry was their only hope to defeat Voldemort -- and it was also the only way for Snape to unquestionably prove his loyalty.
So with that Vow, Snape's life was bound to Harry's, and has been ever since. If Harry dies, Snape dies. And that is why, right from the beginning, Snape was always raging at Harry for taking risks and trying to keep him out of the way of trouble (even if it meant having him expelled from Hogwarts) -- not only because he had a healthy sense of self-preservation, but because if Harry died before he could defeat Voldemort, he'd take Snape with him and all for naught.
BUT in Book 7, Harry has to die so that Voldemort can die. Once they face off in that final battle, "neither can live while the other survives." And the reason Dumbledore was so triumphant when he heard that V. had taken Harry's blood in GoF was that in doing so, Voldemort made that part of the prophecy come into effect and thereby made himself vulnerable in a way he'd never been before. V. doesn't realize it and neither does Harry yet, but their lives are now inextricably linked.
cesario stopped me at this point to object: "I don't read the prophecy that way. More that they can't both be alive at the same time and have any quality of life." But I think that's too abstract, and doesn't really fit JKR's choice of the words "live" and particularly "survive". I think she means literal life, and I believe the North American cover of DH bears this out. Because on it, we see an amphitheatre framed by a tattered curtain, which many fans have already speculated is the Veil -- the mysterious Veil through which Sirius fell when he died, and from which no one has ever returned.
I believe that in Book 7, Voldemort and Harry are going to go through the Veil. And by that measure, they SHOULD both be dead. I think V. and Harry are going to fight it out behind the Veil first, and V. is going to be defeated in some ultimate way, which means that Harry should also die and never be seen again. But Snape will exchange his life for Harry's, thus fulfilling his Vow, completing Voldemort's defeat, and restoring Harry to life.
There is one glimmer of hope for Snape in all of this, however. Because Wormtail ALSO owes Harry -- and Harry specifically -- a life-debt. So there is a possibility that Snape might actually be the character who got the "reprieve" JKR was talking about -- that she found some way to give Snape a heroic and redemptive role at the end, but have Wormtail do the actual life-exchanging part. Still, I am not going to hold my breath on that one.
Anyway whatever I may have got wrong in the above, I think I'm right about Snape swearing that Vow. It explains so much -- especially his CAPSLOCK RAGE in HBP when Harry called him a coward. And his immediate loathing of Harry -- sure, Snape may have sworn the Vow with all good intentions, but how was he to know he'd be protecting a kid who looked exactly like his most hated enemy? Especially since every time he saw Harry, he knew he was staring his own death in the face.
At this point,
cesario mentioned how when Draco was mortally wounded in HBP, Myrtle went for help, but it was like Snape knew that Draco was badly hurt and it brought him at a gallop. So, she said, if Snape did swear an Unbreakable Vow to protect Harry, where was he the first ten years of Harry's life? And why wasn't he totally incapacitated during the Triwizard Tournament?
My response to this was that the first 10 years of Harry's life, he was protected by being with his relatives -- it was only once he got to Hogwarts that things got dicey. The Dursleys may have been abusive, but they weren't life-threatening. Voldemort couldn't touch Harry during that time, was the point. As for the Triwizard Tournament, we don't hear about what Snape was or wasn't up to, until the end. But in any case, I don't think Snape feels when Harry is in danger, or anything mystical of that sort. I don't think there's any kind of mental link. He just has to be watchful, and respond the moment he knows or hears that Harry is in danger.
So in the end, Harry will die, or as close to it as makes no odds. But he'll come back. Which ties in with JKR saying in an early 90's interview that she didn't want to talk too much about her belief in Christianity because if she did, even a child could figure out what's coming in the last book.
Sacrifice.
Death.
And Resurrection.
Now I am going to go and re-read books 1-6 and cry, thanks.