Well, of course there's a possibility that "Margaret" was murdered. Wasn't that a given?
This forced, supposed Carolyn/Burke "romance" is painful to watch. He clearly doesn't mean it, but doesn't seem to be cynically using her, either, unless it's been as an excuse to gain access to the Collins estate, and therefore Laura. He'd just go to the cottage if he wanted to, though.
In the past, there was this mocking, gloating, calculating Burke hidden inside charmer Burke. He loved making Collinses seem foolish, without their realizing it. He'd manipulate and then drop an almost-unmistakeable verbal clue into the conversation, proving his contempt and insincerity, which he knew his victim wouldn't pick up on, just to entertain himself. He did it to help himself gloat, privately. These Collinses are just so stupid, so pathetic, he'd think... Carolyn was his prime target.
Now that's not at work, here. No sex, either. Now it seems as if Burke just eventually goes out with all women he hangs out with for long enough, in a sort of knee-jerk way. He's not even after sex necessarily. Maybe he's just sort of lonely and naive about consequences, but does that sound like Burke?