Commodus had an odd hobby.
More than one. :’) The late Michael Grant noted that the surviving ancient contemporary accounts make it clear that Commodus “had something wrong with his groin”, perhaps a hernia from his birth. He was a big galoot, and was clearly the illegitimate son of a gladiator with whom his whore of a mother had an affair; as emperor he lowered himself by participating in gladiatorial contests, and I think driving chariots.
Eventually he’d bumped off everyone closely related to him (and then some), leaving himself surrounded by servants and others who knew he’d have anyone killed at the drop of a hat, so no one was very broken up about it when he was snuffed. His wrestling partner (not a euphemism, Commodus liked physical competition) was brought into the conspiracy, slipped into the emperor’s bedchamber, and strangled him to death.