[jgorham]

Besides those points, Dumbledore knew that Draco was trying to kill him the whole book, and knew that Snape promised to help him. Harry told Dumbledore about how he heard Snape mention the unbreakable vow as well, yet none of this seemed to phase him in the least.

Also, at the point he died, Dumbledore really had no chance of escape - if Snape didn't kill him one of the other death eaters would have. Seems to me like Snape (and Dumbledore) knew it was inevitable and wanted more influence with Voldemort.