Caligula

The Roman Empire was known for having some batshit mad emperors, and Caligula, the third emperor, was probably the most insane and brutal of all of them. When he became emperor, Caligula was only 24 years old. He only held the position for less than four years before his bodyguards assassinated him.

Caligula was known for his aggressive tendencies; among his claimed offenses were commanding that a portion of the spectators at games he presided over be devoured by wild animals, boasting about sleeping with other men's wives, and having a statue of himself erected in the Temple of Jerusalem. He was also accused of committing incest with his sisters and prostituting them – turning the palace into a brothel and making his horse into a consul.

Joffrey Baratheon has often been compared to Caligula and his style of rule: Joffrey was really the product of an incestuous relationship between a twin brother and sister Jaime and Cersei Lannister, he enjoyed violence and humiliating people – including his betrothed Sansa Stark – and ordered massacres for the slightest infraction. While in the TV show Joffrey sexually humiliated two prostitutes when they were given to him as gifts by his uncle Tyrion and murdered another with his crossbow, it was revealed in the books that Joffrey killed a pregnant cat when he was a young child and when he presented the kittens to his parents King Robert punched him so hard that he knocked out some of Joffrey's teeth.