It was in an ep with Robert Conrad as a guest star. Conrad played suspect Milo Janus. And Columbo outright and bitterly told him (before the last scene) that he thught that he (Janus) killed a sick lady's husband.

Now I remeber the usually easy-going and likable LT loosing his cool and it was unusual for him to loose his temper!!

Actually, in the very first episode ("Prescription: Murder"), Lt. Columbo "lost
his cool" when he accused Katherine Justice of being Gene Barry's accomplice
in the murder of his wife. Great episode, by the way.

He also lost his cool with Dr. Mayfield aka Leonard Nimoy in "A Stitch In Crime."