Because Dumbledore wanted Snape to kill him, because if Snape didn't, Snape would have died because of the unbreakable vow.
And Snape still has a very large part to play in the War Against Voldemort.
And I think the ring might have been already killing Dumbledore, which is why he allowed himself to be pushed to the brink of death so regularly seeking the horcruxes.
Also: Snape was in love with Lily, and defected when he became responsible for her death... Its also why in the first book he looks into Harry's eyes, surprised, and why he hates Harry, who looks just like his father.
The moral of the series this far is that Love overpowers all Evil, and that it is the strongest magic of all.
That's my take anyway.