The Half Blood Prince
my speculation according to probability...
1) It does not seem probable that Dumbledore made a misjudgement about Snape. Yes, he had made mistake but to have the great wizard die for this does not seem likely. He spoke with no shadow of doubt and always with absolute certainty. Given the wisdom he had always shown, we have to believe that he knew something we do not, and he did not, as Harry claimed, simply trusted Snape because the latter acted remorseful. He knew the risk of trusting Snape and he is no fool, as hamlet had said. Unless we're willing to believe that the great DD made a foolish mistake, Snape must have been forced to kill him, against his own will.
Also, Hagrid heard DD and Snape arguing about his "work", which Hagrid believed has something to do with being given too much work, so it seems very possible that Snape was actually rebelling against the assignment, which is to kill DD when circustance requires.
2) Is it becoming of DD to plead? He has always been portrayed as a dignified and fearless wizard so what is the chance that he will buckle & beg for mercy & for his life? Perhaps his beseeching is to urge Snape to complete the assignment, which is, kill off DD & earn V's unwavering trust.
3) Snape is the Half Blood Prince which means book6 is named after him. So Snape should have a very important role to play, one that is more than a murderer and the follower of Voldy. Ok, maybe this is pure speculation, but i think Rowling likes Snape. This Half Blood Prince must have some special surprises in store.