Well, like Kumy, even I'm taking a guess at Greyback.Although I've read everything about him on Lexicon, I still can't understand why he chose to attack Bill. Does anyone have any theories? Thanks!
Apart from the plausible 'Weasley' scenario, I have other thoughts. I agree that Greyback has allowed himself to become completely inhuman and wolf-ish. Therefore, I think he was particularly bloood-thirsty that night. By the way, was the full moon approaching at that time? I know Bill wasn't attacked on a full moon night ... but it's a possibility.
I think Greyback attacked Bill because Bill was brave enough to stand in the way. He wouldn't have cared if Bill were a Weasley or not, he would've attacked with the intention to kill him, only because Bill is on the Order's side.
[/my two Knuts]