Programmed for Murder - S8-E18
Revealing mistake: In the outside shot of the hospital, just before Mort discusses his cough with Doc Hazlet, you can see a sign saying the hospital is located in Ohio, not in Maine. This isn't the only episode where this exact mistake is made.
Answer: In a flashback scene, it was shown that he had a violent temper, so it stands to reason that he finally killed her in a fit of rage.