Look, I'm left with my fingers crossed that he was lying to Bellatrix and Narcissa, and Dumbledore knew something he wasn't telling.

Perhaps, in book seven, we'll find that Snape never killed Dumblebore at all, but only a double that had been created by the use of polyjuice potion. That would redeem book six, at least a bit, but doesn't help at the moment.

I also enjoyed Sirius Black and was horrified when she killed him off. Had she let him live, I'd be more inclined (now that I've thought about it) to believe that Dumbledore is truly dead.