It's real speculation, but I kind of wonder about Eric Mason (I mean before the OBVIOUS thing, MURDERING his wife). Even though his job is all about showing people how to "take charge of their lives," it's easy to wonder how much control she had over hers.
Probably explains why she had an affair with Dr. Mason's best friend, Charlie. Mason even tells Charlie, "To tell you the truth, it wasn't even that good a marriage." Speculating here, but I'd imagine a guy like Mason killing his wife BECAUSE he couldn't control her. Always seemed like he was getting ready to strangle Jo, also probably because he was afraid he couldn't control her, too (regarding the information she had about Charlie and Mason's wife having an affair). Fortunately, Lt. Columbo walks in on them from out of the rain at that moment and he has his little fireside chat with Dr. Mason.