I actually went with the obvious approach. Callie was leaving him because she found out about the murder.
Yeah, I did wonder why none of the characters thought of that as a possible connection between the murder and Callie leaving him a few days later. That was actually my first thought. But it raised the question of why she wouldn't have gone to the police, why she'd leave him for committing murder but still help cover up what he did. Maybe that's why they didn't consider it.