From wikipedia:
Commodus the gladiator
The emperor also had a passion for gladiatorial combat, which he took so far as to take to the arena himself, dressed as a gladiator. This action was considered to be unusual conduct for an emperor by the people of Rome.[8] The Roman people also found Commodus' naked gladiatorial combats as disgraceful and disgusting.[9] It was rumoured that he was actually the son, not of Marcus, but of a gladiator whom his mother Faustina had taken as a lover at the coastal resort of Caieta.[10] In the arena Commodus always won since his opponents always submitted to the emperor. Thus, these public fights would not end in a death. Privately, it was his custom to slay his practice opponents.[11] For each appearance in the arena, he charged the city of Rome a million sesterces, straining the Roman economy.
Commodus did raise the ire of many military officials in Rome for his Hercules persona in the arena. Often wounded soldiers and amputees would be placed in the arena for Commodus to slay with a sword. Commodus' eccentric behavior would not stop there. Citizens of Rome missing their feet due to accident or disease were taken to the arena where they were tethered together for Commodus to club to death while pretending they were giants.[12] These acts may have contributed to his assassination.