It's been said that Mark's father was going to drive with them to North Carolina. Moment of truth approaches. He's in a world of trouble. So what if Mark decided to confess to Lori? She naturally became shocked and angered at the deception and screamed she was "out of there".
Not so fast.
Since Lori was the only good thing Mark had achieved in his life, he couldn't bear her leaving him, so he would rather have her dead than gone.
The rest is history.
sw
I feel so sorry for his family and Lori. I'm praying that she simply left because she found out the truth, but I'm leary of that scenario.
I'm thinking along your lines.
FNC had a former room mate of Lori's on earlier that said there was no way Mark would have hurt Lori, that he loved her too much for that. My thought is that he finally told her and like you said, she got mad and was going to leave him. Perhaps he loved her so much that if he couldn't have her, no one would, so he killed her. (Shades of OJ?)