dude dumbledore is dead. snape is just so badass that when he did the abracadabra instant death he blasted him over the balcony. dumbledore has to be dead in the same way that obi wan kenobi and yoda had to die before luke skywalker could be a man and face vader on his own. this kinda thing is a constant theme through literature.
I'm guessing that Dumbledore was cool with Snape killing him. Mid way through book 6, Dumbledore and Snape are having an argument about something but it never really explains what. I think in this instance the big D knows of the plot to have Malfoy kill him, and he would rather Snape do it than put blood on Malfoy's hands. Plus too then it would look as though Snape has shown ultimate allegience to V, putting him in the perfect position to set up a betrayal and let Harry hit a homerun.
Other evidence that points toward Dumbledore being ok with Snape bliKsmacking him is in his last words. All he says is "Severus please" not, "please don't" but just "please." He could mean, "ok, dude, just do it." *(yah, i know this has been previously stated in the thread)
It's not like Dumbledore is out of the picture... (no pun intended) because they'll be a painting of him in his office at Hogwarts. Or maybe he'll reveal himself to Harry through those little chocolate frog trading cards.
Jam it back in, in the dark.
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