Who was the guessed most cruel leader in ancient Rome?
Caligula AD 37-41 was probably the most cruel of the Roman Emperors. Caligula was greedy and bloodthirsty. He frequently insisted on being worshipped as a god. He was murdered on the capitol steps.
Second to Caligula would be Nero AD 54-68. Nero was greedy, excessive and prone to violent outbursts. He killed Octavia his wife, had his mother murdered, and then killed his mistress, Poppaea Sabina, with a violent kick.
Nero was blamed for the great fire, which started at the Circus Maximus, but then scapegoated the Christians who then were a very minor sect. The Christians were exposed to wild animals, burned alive and crucified on the roads leading into Rome. Their crosses were painted with pitch and lit afire at night to light the Roman roads under Nero's rule. It was under Nero's rule that Peter was crucified upside-down on Vatican Hill and Paul was beheaded along the Via Ostiensis.
After 10 years of Nero's rule he ordered the suicides of 66 members of the Roman Senate. When Galba of Spain declared that he was Emperor of Rome and began to march on the city, Nero ran away and committed suicide June 9, AD 68.
Nero may have been a great musician and artist, but he was probably one of the most cruel of the Roman Emperors.