So Zacharias Smith is descended from Helga Hufflepuff, huh? (Well, if you think Hepzibah (sp?) Smith is related to him.) I wonder if Helga has a similar personality. I want bitchy Helga!
Anyway, on Dumbledore. To me, Dumbledore is a man who will stop at nothing to achieve his ultimate goal, which is protecting the Wizarding World. There is nothing he will not sacrifice. He sacrifices his family, his friends, his morals, his students and, ultimately, himself. He is a man who has been forced to be manipulative for so very long that he no longer knows how not to be. But he is not a Slytherin. He is cunning and clever and sly and ambitious, yes, but his ambition is not for himself but for the greater good, and that is what makes him a Gryffindor.
I believe that he honestly cared for Harry, and that that frightened him, because he is not someone who can afford to honestly care for people. He can't afford for there to be somebody he is not completely willing to sacrifice for the greater good. Especially not if that person is Harry, because, frankly, Harry is kind of a sacrificial lamb.
I also believe that as black_dog says here, he did know that Snape took an Unbreakable Vow and both he and Snape expected Snape to be the one to die. He was not, however, pleading for his life. Because, in that instant, both Snape and Dumbledore recognized that Dumbledore had to die.
Finally, Dumbledore was not a very good Headmaster. For one thing, he didn't become a professor because he enjoyed teaching; he became a professor because Hogwarts is, in many ways, the center of Wizarding Britain.
I can't decide whether I want to write MWPP-era fic or Founderfic. Hmm.