My vote as most psycho goes to Leslie
Williams in "Ramson for a Dead Man". Her husband did everything for her, got her a good law practice, gave up his seat on the State Supreme Court, and even after she made her demands that he let her live a separate life we see that he still maintained an apparently cordial relationship with her, she cold-bloodedly kills him. I am surprised she didn't get rid of her step-daughter when she started threatening her.
In fact, at the end, Columbo basically tells here this when he says that she "has no concience". (What is odd is that Adrian Carcini is somewhat similar, but Columbo stays friendly with him to the end)
My nominations would be Basehart and Blackman in Dagger of the Mind.
Whilst the initial 'murder' is more an accident, they kill Wilfred Hyde White in cold blood, and Basehart is clearly losing his grip on reality by the end of the episode.
Re: Re: Re: Re: Re: Re: Columbo's Most "Psycho" Killers
Abigail Mitchell, while not really "psycho," is pretty scary when she says to Edmund, as she is slamming the safe on him, "Do you really think I didn't know?!"