I'll throw in Paul Gerard in "Murder Under Glass".
He manipulated eveybody in sight; blackmailed members of the Restaurant Developers Association; cruelly poisoned one of these members; and even tried to kill off our favourite detective....and of course Columbo hated him!
Let's not forget Paul Galesko (Dick Van Dyke) of "Negative Reaction." It was just not convincing to me that his wife should have been killed, and in addition to that he destroys the life of and frames an ex-convict who was apparently looking to get his life back on the right track.
I think Rip Torn in "Death Hits the Jackpot" was a grade A tossbag. Usually the victims provoke their own murders in some way but this murderer kills his own nephew to steal his lottery winnings.