Alright, so here is my theory on Snape:
Snape is good. Remember in the beginning chapter when Snape and Narcissa make the unbreakable vow for Snape to protect Draco and to finish the job if Draco doesn't? Well, I think Snape was able to do that because he and Dumbledore had already made an unbreakable vow for Snape to also protect Draco and to UNDER NO CIRCUMSTANCES ALLOW DRACO TO KILL HIM and so Snape had to promise to kill Dumbledore, should the situation ever arise. And he was able to do this b/c nothing in the second vow contradicted anything in the one he made with Dumbledore.
Also, I think Snape is a double agent death eater/member of the order b/c he can do occulemency. We all know he is very accomplished at this...perhaps even so accomplished that even Lord Voldemort can not penetrate his mind! He can meet with the death eaters (block his mind in case any of them are legilimens) and report back to the order.