I find the murderers in the newer episodes to be more annoying. I think they were puposely written that way. It's hard to beat the two frat boys in "Columbo Goes to College".
As far as the older episodes, I think the writing focused more on making the murderers arrogant and boorish rather than just plain irritating. But I agree, Beth Chadwick was pretty annoying. Roddy McDowall was a bit irritating too.
In some ways, Alvin Deschler. Don Gordon (who was always great a "hard luck" characters) really makes you believe in him as the "ex-con trying to go straight." And that makes him so grateful to Paul Galesco that he's naive about him, and look where that gets him.
Also Gene Stafford, for one of the same reasons. Philip Bruns was great at playing a "put-upon" character on the show MARY HARTMAN, MARY HARTMAN, and Gene is a really put-upon character. Ruth admits it about herself by saying "Between Milo and me, he didn't have anything BUT trouble." And of course trying to do something about it gets him murdered. So I don't have to dislike Milo Janus as much as some people do to be bothered about that one.
William Shatner irritated me the most. In both episodes, "Butterfly in Shades of Grey" and "Fade into Murder." He is so arrogant and self-centered, I don't even watch those episodes.
Just thought of another total creep. Dr. Collier. Murdering the very mentally ill Nadia Donner was very bad. Especially the way he did it.
Jarvis Goodland in the Greenhouse Jungle! His whole attitude to everyone, his nephew, his wife, Columbo!