in my opinion, KAY Freestone had two motives for killing Mark. One is that she was a scorned lover. And secondly even though Columbo said that people usually don't kill for something as trivial as a job, I think she did. When she found out that Mark was not going to let her have his job, she offed him.
M. Janus almost fits into the same category He needed complete control over the gym and his embezzling operation so he kills Gene Stafford then makes a pass at his ex wife?? While still having his sexy secretary on the side What a creep.
I had to think about this one for a little bit but this is what I came up with.
In A Deadly State of Mind, Dr Collier kills Karl Donner so he could have Nadia all to himself. Then later when her story starts falling apart, the one person that he wanted has to be killed so as not to frame Collier
But Columbo was suspecting him anyway by that point
Another tricky one is Mark Halperin. Obviously he wants Margaret's money so he can spend it, but when he complains about her "bleeding heart" causes, he sounds touchy enough that you can believe that that's a motive too. One of the most "shabby" ones, but still a motive.