It is an interesting post Carl. I think Eric Mason in How to Dial a Murder pushed the four murder or potential four murder mark, but I could be wrong. He killed the primary victim with the dogs. I believe it was mentioned that he was responsible for his wife's death. He briefly had his hands around the throat of the female boarder(Kim Cattrall), when Coulmbo knocked on the door an interrupted. And of course, he attempted to kill Lt. Columbo. What an idiot!
You would think by season seven the murderers would realize you can't beat the Lieutenant!

He might have murdered two people, but it always seems like Rodger Stanford sets the record for the most people he plots against altogether - along with the two murder victims, he tricks his Aunt Doris, he uses Betty to frame David, he frames Quincy as a blackmailer (both of those AFTER they're murdered) and he frames Everett. He even tricks the police into thinking his trying to hide those photos makes him the good guy! And before the beginning of the story, there's the way he evidently invents some of that "corporate bad feeling."

A little bit off-topic, but Rodger Stanford was on the absolute top of my creep list