I understand what everyone is saying about Lizzie already. But I have a new piece of evidence that it could have been her that killed Karen and David. So I'm rewatching Season 4 while doing some college work. It occured to me that Lizzie was upset that Nick was destroyed. Carol gave her a knife and told her she was weak! It would be safe to assume that Karen and David destroyed Nick, so why wouldn't Lizzie kill them as revenge? Then Carol caught Lizzie maybe, since it would be her new role to know where Lizzie and Mika are. Carol burnt the bodies to protect Lizzie's bad kills. Could we assume that Lizzie did a messy or sloppy job?
And since she fuelled Lizzie by giving her the knife and motivation to prove herself, wouldn't Carol feel it's her fault anyhow? I understand people have said that Lizzie wouldn't know why Karen and David were seperated but it's obvious when you got a prison full of people gossiping about what's going on. I mean come on what else is there to talk about if you were a red shirt in the prison?
Also Carl telling Rick that Carol is teaching them to kill and Rick saying he wouldn't stop her, is the perfect plot for Rick to have made another error...
Anyhow prove me wrong. Poop on my speculation. Fault my logic. Go on...