Why didn't Voldemort die?
We know that Tom Riddle pretty much vanished after graduating Hogwarts, pursuing dark magics and means of insuring his own immortality. I for one think Hagrid got it right when he said Voldemort wasn't human enough anymore to die. Whatever-it-was he did allowed him to survive the backfired spell.
Okay, that's all well and good. In fact, it just paraphrases what has been said by others. But here's the real point--don't you think he's going to try and do it again? Might not that be a really major part of the story in HBP? Whatever Voldemort is plotting, methinks it is something really horrible and dark.
Why didn't Dumbledore try and kill Voldemort when he had the chance?
Okay, my first instinct is that to do so would also kill Harry at this stage--and that Dumbledore needs to somehow protect Harry first. If so, maybe the plot of HBP has to do with not only preventing Voldemort's anti-death plan from succeeding, but making sure it is Harry who gets the benefits of same. Thus, Harry will survive Voldemort's death. Aong those lines, the Whatever-It-Is must be something that cannot be replicated again for some reason.
That's all well and good, and perfectly plausible as far as it goes. But here's something else maybe--because Dumbledore wants to defeat more than simply Voldemort. He wants to bag most if not all of the Death Eaters, and his plan absolutely requires Voldemort to do something he hasn't done yet.
Mind you, these two are not in any way mutually exclusive. Heh heh.
"O let my name be in the Book of Love!
It be there, I care not of the other great book Above.
Strike it out! Or, write it in anew. But
Let my name be in the Book of Love!" --Omar Khayam
maybe dumbledore has seen his own death, and that harry is the only chance to stop voldemort, but hes not ready, so he has to keep him alive, till he(dumbledore) dies and leaves harry to take on Voldemort