Whenever there's that kind of reason, I wonder the same thing.

Did Nelson Hayward want to lose Harry Stone, even if it hadn't been for Linda?

Did Ray Fleming want to lose Carol, even if it hadn't been for Joan?

And I guess there are many others.

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.