I think you are right. There was a scene where Locke turned into the black smoke and killed some gaurds/"others". after he turned to Ben and something like, "sorry you had to witness this."

Do you guys remember the scene last year where Jacob was sitting on the beach with a dude in a black tunic? I think he is the evil guy inhabiting locke.. from the setting it looked like the 1700's based on the pirate ship floating in the background.

I'm embarrassed I know this much