I agree. Most of them are cold-blooded - Ken Franklin, Bart Keppel, Great Santini, Milo Janus, Eric Mason come to mind. Others do things in the heat of the moment - Brimmer in "Death Lends a Hand", Vanessa Fowler in "Columbo Likes the Nightlife". I think the killers who did their deeds in the heat of the moment would be more likely to have something of a conscience.
Carsini's murder was not pre-planned, either, but when he confessed he said he said there's no remorse attached to it. Carsini had been faced with the loss of the only thing he ever loved and reacted out of fear and anger. Then, the rest of his world began to fall apart. First, Columbo revealed that all his priceless wines had spoiled, making these huge investments worthless and then as Karen, ("quite the little Iron Maiden") began to turn the screws on him. He was caught and knew it, but really had no where else to go. He was really most concerned about the winery ("Who will take care of this when I'm gone?").
Others - such as Dr. Mark Collier ("A Deadly State of Mind") and Vivica Scott ("Lovely But Lethal") commit crimes in the heat of the moment, but later commit a second murder in a heartless manner, which makes me believe they really have no conscience and would never confess.
Thanks for the feedback. If I had to pick the most likely murderer whose conscience would eventually get to them it would be Ruth Lytton, especially if her niece were convicted and received life in prison. She seemed very cold blooded in how easily she killed her brother, but I still think it would eventually have gotten to her.
Ruth Lytton is an interesting case. She had a logical process to her where she seemed to want things to be done for the right reasons. It's why she turned off lights all the time to save money (which ultimately led to her capture), felt Columbo should tell Janie "it wasn't true" about her poisoning Janie's father with the chamomile tea and why she felt it was the right thing to kill her brother and Shaeffer. As to whether she would have let her conscience get to her - she did frame Janie by planting the belt buckle. (Perhaps she knew the evidence wouldn't stick). Would she confess to keep Janie from going to jail? Probably, because that wouldn't be the "right thing" to do. If anyone should be punished, it should be Ruth, as she was the one who committed the crimes.
Interesting topic, Irene!
In Lovely But Lethal, Viveca gives a pretty sad look as Shirley (who's her only DELIBERATE killing) drives away. It's hard to tell from that look just how bad she does or doesn't feel about it, but at least it's there.
I always found her goodbye to Shirley to be more of a sarcastic “little do you know this is a FINAL goodbye” look. It didn’t seem to have any remorse attached to it to me. I find many of the 2nd killings of the blackmailers or the people standing in the way of getting away with murder always turned those murderers into a person who feels justified that the 2nd victim brought it on them-self.