I mean come on, I know Beth is meant to be innocent and all that, but this season she seems to of gone kind of dark... I'm not saying for sure that she is the mysterious killer. But I think it sort of makes sense and it would be quite unexpected plot twist. I mean I love Beth but imagine that twist! But other people I could see it being:

Bob: I'm not that for into the comics, but isn't it a medic who betrayed Rick and the prison for the Gov? I don't think it's him, but it could be!

Lizzie: She seems to be a bit crazy, and she could be feeding the walkers because she thinks they're normal people.

Morgan: He knows where the prison is, he seemed to try to point out to Rick eveeryone dies.. He burnt bodies aswell.. Maybe hes still trying to "clear"?

Anyway thats just my ideas, tell me what you think?