I think opportunity PLUS the inherent motivation of the character is the key to placing Columbo in harm's way. Milo Janus is a ruthless bully, which is why I've always wondered why he didn't try something. His motivation is to destroy anything or anybody who gets in his way. He had the motivation, the means AND the opportunity - they were alone in his office. As a contrast, Dr. Barry Mayfield (A Stitch in Crime) is another guy, who given the opportunity, would have surely tried to kill Columbo. However, he was trapped in the hospital when Columbo nailed him, don't think there was much he could do. He had motive, means (I'm sure there was some available drug handy), but not so much of an opportunity.

As for Adrian Carsini, I don't think he felt anger towards Columbo. There was that mutual respect thing going on - I don't think he resented Columbo and Carsini isn't the ruthless brute like Milo Janus. He had means (shoving him off cliff), opportunity (at the cliffs - nobody around) but no real motive.

(I just remembered Dr. Eric Mason (How to Dial A Murder) who tried to kill Columbo with his dogs.)

Ellítt Blake had Columbo at the guillotine with his head in the line so to speak. He DID try to kill him although Columbo took a huge gamble in switching up the collar That took cajones. I mean losing a finger is one thing but a whole head? Anyways he outsmarted Blake in the end anyways

Ahh, yes, forgot about that one.

Swan Song. Tommy could have easily taken out Columbo up on that hillside all alone in the dark. He even asks Columbo if he's afraid of being alone with a cold blooded killer.