My theory is she confronted him on Friday but he smoothed things over with her by saying the school had a mixed up the names or something. He knew he didn't have much time left before she exposed him so on sunday when she fell asleep he killed her. I also believe he wanted to continue this make believe lifestyle and he knew he couldn't continue it with her. I also believe there may be more to the story and that mark may be involved in illegal activities that he doesn't want family to know about.
He might have thought he could keep the lies hidden...but I think it goes deeper...I don't think he wanted to live with her knowing he was not what he pretended, that her not pursuing her graduate studies was on hold because of his fantasy college attendence, graduation and fantasy med school acceptance...
Her knowing eyes were too much and so he closed them...I read another poster say.. "The sound was too loud and he had to turn off the radio"...His "double life" is yet to be revealed... whether he had other secrets...
He has a personality disorder but he is not insane in the legal sense IMO...In Utah the usual insanity plea is "guilty but insane" with a person serving remainder of sentence in jail if they get well in the psych hospital...I don't think he can plead insanity.
I read in a article yesterday that Lori told a friend that she was leaving Mark. And the police were looking for a friend that Lori visited at 10:30 pm that Sunday night. The friend had just had a baby. My theory is that this friend went to the police and told them that information. The police say that they know the motive.