Dead Letter - S6-E6
Factual error: Unlike the Connie Kowalski character in this episode, real insurance investigators are not trained in arson investigation. Insurance investigators search for evidence of fraud relating to insurance claims. Arson investigators are professional firefighters who have had extensive additional training in arson investigation.
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.