This was what I was wondering too. It all seemed wired if I'm honest. Like his reactions to Mary talking to him like he instantly wanted her dead and out of the picture didn't feel like the Jack we were used too. Jack might do the wrong thing, but he's not an instant murderer. It feels oddly out of character for him just to obliterate Mary. Jack had feelings for Mary, and did feel there was something there for her, and the way he killed her just felt like a different person was doing the killing. I remember when Sam was soulless and he was horrible trying to kill Bobby in the cellar, and doing a deal with Balti to keep his soul in. These are actions of people who are not in their right minds. Jack doesn't know what his mind is doing. Was Lucifer acting that act of murder on Mary, and not Jack?