Re: Killers you like when you aren't supposed to, and vice versa.
One character that seemed widely liked by fans (and in the script itself) is Tommy Brown from Swan Song, played by Johnny Cash. I think it's Cash's charm and identity that blurs the real truth - that he is perhaps the most vile of the whole bunch.
In the episode, he is seen not only killing the religious leader who secured his release from prison, but also the quite innocent underage girl he was repeatedly victimizing. Double murderer. Serial rapist. Even finds time to de-fund a charitable undertaking + sought out a fresh, new young victim while plotting it all. Yet he's considered one of the most likable ones. I always find that funny.
Is there a more despicable Columbo killer I'm forgetting?
Re: Killers you like when you aren't supposed to, and vice versa.
For me it was:
Tommy Brown - How can you not like Johnny Cash and his music? If the part had been played by anyone else, I would've despised him for killing the innocent girl.
Abigail Mitchell - Just a sweet, loveable little old lady exacting sweet, deserved revenge on the death of her only daughter. Eye-for-an-eye, I'm OK with that.
Adrian Carsini - This was one of the first Columbo episodes I ever watched, and the first one I saw where Columbo seemed to genuinely befriend the suspect. Poor Adrian!