I've posted this in other forums before and thought it would fit here. I am almost done watching all 60+ episodes of Columbo, and stand by my position since episode 5, that he's not human. The way he locks in on a suspect, the furtive way he moves around the room, it's like he's an eldritch arachnid being wearing a rumpled skinsuit. It would explain the few times he got angry. He was "molting" and shedding his exoskeleton, limbs growing underneath is coat, causing agony.
Also, the man is a stalker, worse than Freddy, Jason, Michael and and Chucky combined. Instead of brutally murdering the killers, he's psychollogically tortures them (that probably makes them more tasty to the arachnid Mrs Columbo), pretending to be a shambling old man, showing up at impossible places, or when the killer looks out the window, Columbo looks up at them from the shadows and waves. This happens a LOT.
It makes me wonder if the true ending we don't see is Mrs Columbo coming in to feed once the killer has been driven mad by the mannequins of death and Columbos incessant trolling. Of course by the end of his case, he goes home to where he sleeps in the loving cacoon of Mrs Columbo.
Edit: Forgot to add that he was written as a manifestation of justice. He "comes from nowhere and returns to nowhere."