[Cyclopean Squid]

I was thinking....maybe the reason Dumbledore is so trusting of Snape is that they had sworn an Unbreakable Vow. The condition for letting Snape back into the fold was for him to agree to do whatever Dumbledore asked, at the price of his life if he refused. Therefore, Snape was bound by two vows--his one to help Draco and his oath to obey Dumbledore. After learning of the Draco Vow, Dumbledore would have come up with a way to fulfill both while advancing the plan to defeat Voldemort. The solution was to allow his life to be taken in circumstances of his own choosing.

[Gardener]

Oh that's a good theory and would explain it well!!

Now, IF Snape is still working for the OOTP, who is he reporting to with D dead (or everyone thinking he is)? Is McGonagall in on it? Is he going to report to anyone at all or is he a deep agent put in place to be ready at the final battle?

[SauronOfMordor]

Keep in mind Dumbledore's blackened and unhealing hand. I think what's going on was D was in pain and dying anyway because of his wound. Thus, he could ask Snape to hasten the his inevitable death, in order to advance the plan