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?