My theory is that Dumbledore was already dying (both from the potion and from his earlier brush with the horcrux). He allowed Snape to "murder" him, because Snape had (maybe unwittingly) made an unbreakable vow to do so. Thus he preserved Snape's life and placed Snape in a position to help Harry later on.

In this light, I see it more as Dumbledore sacraficing himself for the greater good. It was painful for Snape to do it, but it was an act of loyalty on his part to trust Dumbledore's plan.