OK, this is a bit far out and freaky and I haven’t tried to state it succinctly before, but here goes.

First, I dont’ think JK has spent countless hours developing a children’s story about the clash of good and evil to have good done in in the end by evil, I see several possibilities but they revolve around tried and true literary themes.

If the story represents one go round in a Cycle Of Clashes Between Good and Evil, then Dumbledore previously had a mentor, just as he is mentor to Harry. His triumph over “the dark wizard Grendelwald” or whatever it is that it says on his chocolate frogs card was his life turning point, changing him from hero to Merlin-like figure. Likewise the former dark lord’s defeat cleared the way for Voldemort to become the current dark lord with Snape as his protege in waiting. Because Dumbledore understood this cycle, he recruited Snape to secretly defeat Voldemort so that Snape can become the new Dark Lord for the next cycle. The conflict which results in the defeat of th Dark Lord will transform Harry into the next mentor for good, who will have to find the hero who can defeat Snape when his powers are fully realized. (how slytherin-y this would be, Snape going to any length, even the doing of good, to attain his evil ends).

If this is the Story of the Self Sacrificing Hero (think Christ themes), then Harry must die for the defeat of the Dark Lord but he and/or Dumbledore will somehow defeat death through the power of love. His “spell” to kill Voldemort entails his own death, but somehow, he emerges beyond death showing us that the “power the dark lord knows not” is the ultimate magic.

The Last Missing Piece theme - here we have been told time and again the four houses must unite to save Hogwarts (read defeat the Dark Lord), but where is the help from Slytherin? My answer is that Slughorn finally turns the tide. Here is where comes to play the importance of Harry having “lilly’s eyes,” when Slughorn overcomes his own natural incinations, nurtures harry through the last hurdles, and brings along the reluctant Slytherin students at Hogwarts into a “unity” that makes the forces of good which defeat Voldemort in the end a ‘whole, greater than the sume of their parts.”

OK, clearly I’ve been giving this way to much thought. =P