Within the later tradition, the most obvious evidence of imperial madness is offered by Gaius’ imperial divinity. Suetonius has an origin story for Gaius’ development of divine aspirations (Gaius 22). He had tried ‘pius’, ‘castrorum filius’ (son of the camp), ‘pater exercituum’ (father of the army) and ‘optimus maximus Caesar’ (best and greatest Caesar). Various of the client kings came to Rome to pay their respects. At dinner, they fell to arguing about their titles and the distinguished nature of their ancestry, as kings do. Gaius became irritated that there was no obvious way of showing his superiority. He thought of adopting the title of ‘rex‘ (king), but was unconvinced and decided to associate himself with the gods.
The chronology of what happened next is unclear, nor, indeed, can we be confident about how much of what follows is true. Famous statues of gods were brought to Rome be decapitated and a likeness of Gaius to be placed on their shoulders. His palace was extended and the entrance remodelled to encompass the temple of Castor and Pollux in the Forum. Gaius then appeared between the two gods to receive worship. He was called Jupiter Latiaris, a god who had some association with human sacrifice. Priests were established for his cult and rich citizens offered sacrifices for him (of exotic birds). He built a link between his palace and the temple of Jupiter Capitolinus, where he had some sort of residence. He engaged in conversations with Jupiter. He was seen inviting Selene (the moon) to his bed. Lucius Vitellius was supposedly asked by Gaius whether he could see the goddess in his presence and saved himself by claiming that only gods could see other gods (Dio, 59.27).
Dio (59.28) repeats many of the same stories, but adds others. A temple to Apollo under construction in Miletus was converted to the worship of Gaius. Two temples to Gaius were built in Rome itself. He dressed as Neptune, Bacchus, Apollo, Hercules, Diana, Juno and Venus (Dio 59.26).
Towards the end of his reign, he decided that he would have a statue of himself inserted into the Temple in Jerusalem. This would have been a desecration of the most holy site in Judaism and would certainly have caused an enormous rebellion. Fortunately, he was murdered before he could force his will on the governor of Syria (Philo, Legatio ad Gaium; Josephus, Antiquities 18.8; Anti-Semitism)
From the Augustan period, there had been widespread emperor worship in both the East and the West. Worshipping a human was not common in antiquity, but figures such as Romulus, Aeneas, and Hercules, and heroic figures could receive cultic offerings and possibly even temples. For Augustus, official cult had been restricted until the death of the emperor. Drusilla had also received divine honours when she died. The boundary between human and divine was thinner for the Romans than it is for most modern religions. Treating the emperor as if he were a god was an extension of a pre-existing practice, but was not revolutionary.
What cult provided was a set of rituals which would mark the subordination of all others to the emperor. If the emperor was looking for a way of asserting his status, as Suetonius claims, divine honours were a possible method.
The problems are, of course, in part religious: the emperor was a human and not a god and to claim to be a god offended conventional morality. In part they are about the relationship between the emperor-god and those around him: how does one relate to a god? The benefit and problem of divinity is that it referenced an absolute and uncontrolled power. But if such power was beyond all control, how could one live with it? The divinity of the emperor suggested madness and the solution to his madness was assassination.