[DisneyFan25863]

Spoiler:

Ok, I was thinking about this last night. Dumbledore said earlier that he was not afraid of death, right? It was either in the 4th or 5th book, I don't quite remember. Well, if he's not afraid of death, why would he be pleading for his life? He was prepared to die, we already knew that, since he was a member of the Order. He probably, even after the potion, still had enough power left to pull one of his tricks like he did in his office before the Auror's in book 5. So WHY would he plead? To Snape, of all people?

Maybe his death was planned.

Perhaps he realized that the only way to find the locations of all the Horuxes was to have Voldemort tell him directly. And who better to tell than Snape, who is already in his inner circle. Of course, Voldemort would only tell Snape if he trusted Snape 120%. How to do this? Well, Snape would have to preform a huge duty. Either kill Dumbledore, or kill Harry.

So, I believe, Dumbledore intentionally asked Snape to kill him. He knew he was going to die, thats why he gave Harry the order to leave him behind to save himself if he had to. Dumbledore wasn't pleading for his life, he was pleading to Snape to kill him. Thats why Snape didn't just kill Harry on the lawn. Thats why Snape didn't take out Hermione and Ginny and Flitwick when he had the chance.

In addition, I believe that Snape ensured Harry's position as an Auror. By making sure Harry had his copy of the potions textbook, and by leaving the post of Potions, he made sure Harry would pass his N.E.W.T.S, allowing him to become an Auror.

It's all staged, IMHO.