I wondered what happened to Luke from the start. He had some seeds of unpleasantness before, but nothing like what he became. I don't think he'd raped before (although he came close with Janine) but he was a user and a total pariah. Even Kirk wondered what happened to him, as he said Luke was as against what Mark did as the rest of them. Maybe that is a part of the puzzle, or maybe Luke just went a little crazy, as he said he did.
Is it significant that this seemed to happen after Mark left?