I just finished this book... First I don't believe that he is really dead. I think Snape killed the part of Voldermorts soul that was in the locket. I think the locket that was left was a decoy.
Malfoy was suppose to kill Dumbledore, not Snape.. if Snape was truely evil he would have followed the orders of Voldermort...I think he knew what to do by reading the thoughts of Dumbledore as he did with Harry. I believe we will see Dumbledore, Snape and Malfoy in the final battle against Voldermort.
Now I'm going to read this thread and see if anyone else thinks this way.
That is my impression. He may have also killed Dumbledore, but a murder was committed, and that was the requirement for making the soul splitting thingy.
Can anyone say Star Trek III?
I don't know how an Unbreakable Vow works, but I think Snape had to step in, in part because of the oath he'd made with Narcissa.