Michael Landon - excellent choice! He would have been a great Columbo adversary.
A few other choices:
Henry Fonda (think about what a terrific villain he played in "Once Upon a Time in the West")
Sandy Duncan (she was solid in "Law & Order")
Dolly Parton (Johnny Cash pulled it off, I think Parton would have, as well)
John Ritter (maybe he got fed up with his roommates; I think Ritter was a good actor and would have been a great guest)
Robin Williams (I can imagine a Joe Devlin-esque type of role for the incredibly multi-talented Williams)
Alan Hale Jr. Now there's a guy who could definitely be a good murderer. He's got the "look" of a great killer.
Speaking of a Joe Devlin type, how about Scotty from Star Trek? James Doohan? I think he would be good.
As for the women:
It's a bit of a stretch of the imagination, but how about Kate Jackson? One of Charlie's Angels could definitely not be an angel. He was also on a police show, I think it was called The Rookies?
Karen Fielding. Imagine if Carsini does not get caught. Maybe one day Karen Fielding has had enough of his BS (now as his wife) and decides to off good old Adrian. Just an idea.
Since I'm very fond of the first seasons, I keep thinking of early ' 70s TV actors (not that actors of all kinds didn't play the murderers).
One that just occurred to me is Bill Bixby. He had such a "nice guy" image that he would been a big surprise (like a lot of other actors who played Columbo murderers). He also had a "cool" way about him like several of them (Robert Culp and others).
Heres a good one I should have included. Bob Dishy LOL. Can you only imagine what kind of a meticulous murderer he could have been?
---"This might be stretching it a little bit, but what about Andy Griffith? All of a sudden the Typecast Sheriff goes to the dark side."---
Steve, if you'd really like to see Andy Griffith go to the dark side, take a look at his TV movie, "Pray For The Wildcats". It's available on YouTube.
Andy plays a despicable, lecherous bully. Talk about "against type".
I saw this TV movie when I was a kid and was shocked that this was good ol' Sheriff Taylor. The movie also features William Shatner, Robert Reed and Angie Dickinson.
Some viewers may find it campy. If you're able to separate the actors from their most famous characters, then you should be able to find this movie enjoyable.
I've always felt that Larry Hagman would have made a great murderer. He would have been coming off his role as lovable Major Tony Nelson in I Dream of Jeannie and it would have been a precursor to his role of JR Ewing on Dallas.
How about Raymond Burr?
He could have defended himself!
I'm liking the Andy Griffith and Larry Hagman ideas - too bad they weren't reality.
Fred Gwynne (Herman Munster) would have been terrific as a villain, as well. His imposing size would have been enough, but coupled with his voice and presence, he might have even been able to scare Columbo.
The same with Robert Middleton, who played the victim in Double Exposure.
James MacArthur (Danno) would have bern excellent.
C o c k s
Andy Griffth played a murderer in a Made for TV movie with Johnny Cash.
I haven't seen it in some time, but the odd thing is that Johnny Cash's character is a sort of low-key Columbo type cop. Maybe not comical, but like him in some other ways.