Am I the only one thinking that Sam does the killings? He seemed pretty off to me in the episode 5, and 4, coming to Dawn's house to act like a perv.
Plus, remember how he "doesn't think like other people" and hangs out by himself a lot. And if the killings were vampire killings, then the body would have been drained of all blood. And wasn't.
Sam's the bad one here ;-)