I think at that point ending of the marriage would not have done it either. He had lied to so many people, not just to Lori, but the entire family. Maybe he killed her to create a disturbance to take the heat off of him and his lies. Only because he is so warped anyway, he created more of a situation that placed the attention "entirely" on him. Well come to think of it, maybe that was his actual plan. He seems to desire attention. If you look at it...he had to create a disturbance while everyone was focusing thier attention on finding Lori. Maybe that was his prime goal all along.
All in all. The guy is a total freak.