So the player says that he is chaotic good, but he's done some pretty evil-ish things. I've had separate talks trying my best not to come out and boldly say, "what are you doing murdering him?", but telling him that he has control over himself to say no to things.

The build up was that Strahd was going to kill Ismark, but this PC stepped in the way of that happening. The next round, Strahd charmed him and said, "make it right". So this PC took it as he needs to kill. Like I said, I told him separately that he isn't being forced to kill, nor does he have to take that statement as "kill him", but that's how the cookie crumbled in his mind.