I\'ve always felt that Larry Hagman would have made a great murderer. He would have been coming off his role as lovable Major Tony Nelson in I Dream of Jeannie and it would have been a precursor to his role of JR Ewing on Dallas.
Weird you mention Larry Hagman, because wasn't he in a McMillan and Wife episode that was later converted into the Columbo episode "Uneasy lies the Crown"?
I'm liking the Andy Griffith and Larry Hagman ideas - too bad they weren't reality.
Fred Gwynne (Herman Munster) would have been terrific as a villain, as well. His imposing size would have been enough, but coupled with his voice and presence, he might have even been able to scare Columbo.