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.
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.