The question seems less and less to be whether or not Dumbledore is really dead or that how he died wasn't totally expected. Rather, the question is whether or not Snape truly betrayed Dumbledore or whether or not killing him is part of some "master plan."

If you re-read the few paragraphs from where Snape barges onto the scene through where Dumbledore is killed, the words are chosen quite carefully. Nowhere does Dumbledore actually ask Snape to NOT kill him. All he says is "Severus." and "Severus ... please." Dumbledore may in fact be asking Snape to kill him. And if so, he intentionally wanted Harry to witness it.

Chew on that one for a bit.