I believe that Snape and Dumbledore had planned Dumbledore's death. Look at this conversation between Hagrid and Harry from HBP:
Hagrid:I-- well, I was coming out of the forest the other evening and I overheard them talking-- well, arguing. Didn't like to draw attention to myself, so I sort of skulked and tried not to listen, but it was a--well, a heated discussion and it wasn't easy to block it out.
Harry:Well?
Hagrid:Well-- I just heard Snape saying Dumbledore took too much for granted and maybe he---Snape--- didn't want to do it anymore--
Harry: Do what?
Hagrid:I dunno, Harry... anyway, Dumbledore told him flat out he'd agreed to do it and that was all there was to it. Pretty firm with him.
Dumbledore must have already been dying because of his wound from Marvolo's ring, and so he had made Snape promise to kill him. Like DoubleWide said, that's why Dumbledore paralyzed Harry; he knew what was going to happen and didn't want Harry to interfere.
What do you guys think? *is prepared for criticism and blackmail*