I don't think Eric took Bill. There's no way he would've used silver to do it. Eric has seniority over Bill anyway. I'm sure it wouldn't be too hard to figure out since books all the way through season 9 I think are out anyway...I'm still working my way through the first book though which is roughly Season 1.
I'm guessing it wasn't a vamp that took Bill and probably not somebody from Bon Temps since they are almost all recovering from Maryanne still. Maybe it would be somebody from the Fellowship of the Sun but I think they are done as a major character. I'm leaning more towards somebody completely new.
It is very possibly a new character, but I wouldn't rule Eric out completely. Yes, this does seem like an obvious path, but the facts are there - Eric promised to take care of Bill, and the perp was wearing gloves, so I imagine they could handle silver even if they were a vampire.
