I think Dumbledore wanted to die because he has been aging more and more since the Philosopher's Stone was destroyed (although maybe that didn't have any connection, but it's been repeated that he was aging noticeably), and after the irreparable injury to his arm, it was only going to get worse. Why he didn't amputate and get a replacement like Pettigrew, though ... is probably one of those questions you leave to the suspension of logical thought.
Hopefully now Harry will wake up and come to some understanding with Snape, but since Snape won't answer to anyone especially now that Dumbledore's gone, maybe not. It would probably actually be less interesting if he did, just another of the morals for the kiddies... but since they are kids' books, and they are acceptably well done, I'm not really complaining. So that's only my half-expectation, and I don't really have any others.