Harryisdbest (or should I call you Sherlock Holmes!)- Good thinking mate! You really can explain things logically. But I think Dumbledore is dead and he and Snape planned it all.
Remember, Snape and Dumbledore were arguing and Snape was saying that he didn't want to go through with it.
Well I think they were talking about Snape killing Dumbledore.

A friend of a friend of mine me JKR. That in itself is pretty cooool but wait..there's more! She asked JKR about all the stuff that happened in the books about Snape etc. ANd JK said that it's not as it seems and you need to look beneath the surface.
Hmmmmm. That tells me one thing. If everything really is as it seems in the HBP then Snape is guilty. If everything is not as it seems, which is correct, then Snape is not really a deatheater.