Gaius was eventually assassinated in a conspiracy led by officers of his praetorian guard in league with members of his household. In retrospect, the question is not why Gaius was killed, but why he was not killed sooner.
Conspiring Against the Emperor
Planning to kill an emperor was a risky business. Even so, one might have expected that the oppressed senators would have moved against him, as their predecessors had moved against Julius Caesar. There is a tipping point in imperial-senatorial relations at which the most powerful decide that it is more advantageous to remove an emperor rather than keeping him in place.
The praetorian guard existed to protect the emperor. Officers of the praetorian guard derived power and status from the emperor and a more tyrannical emperor depended more on his guard, as we see with Tiberius. It seems that Gaius worried Callistus and the prefects of the praetorian guard by taking seriously baseless accusations against them (Dio 59.25). He also humiliated Cassius Chaerea, tribune of the guard, by intimations of effeminacy (Dio, 59.29; Sueonius, Gaius 56).
The praetorian guard and the household had direct access to the emperor and were thus in the most immediate danger. They did, however, have the opportunity and the praetorians were armed in the presence of the emperor. A paranoid emperor would know that the people with most opportunity to kill him, were those closest to him.
But those people also they had most to lose from a change of emperor. A new emperor was unlikely to trust those who killed his predecessor. To survive the conspiracy, Chaerea and his leading accomplice, Sabinus, needed to be confident that they had some political backing. But if you sought political backing before you killed the emperor, you risked the secret being revealed. Everyone involved in a conspiracy had to be gripped with fear that someone else might fail to keep the secret. By the end, the conspiracy appears to have been widespread. Josephus gives us the most detailed account and it is clear that many of those closest to Gaius were in on the difficult attempt to get him away from his Germany bodyguards and to a place where Chaerea could strike.
The conspirators faced extraordinary risks. Yet, they acted. There was clearly courage in so acting. But we may also wonder whether they had any alternative. Chaerea struck him in the face. Other joined in. As with killing of Caesar, many wanted Gaius’ blood on their knives (Josephus, Antiquities 19. 1-14)
Aims of the Assassins
Gaius was stabbed as he left the theatre. Sabinus and Chaerea were at the forefront. Their first intentions was to kill Gaius.
They also killed his wife, Caesonia, and their infant daughter. It seems likely that they sought out Claudius, but did not find him. It seems as though they intended to end the Julio-Claudian line of emperors.
The consuls moved quickly. They transferred the money from the various treasuries of Rome to the Capitol, where is could be easily guarded. The Roman state kept cash. Even if Gaius had spent a lot, moving any large amount of cash was difficult and required planning. Money was a source of power. The speed with which they secured this crucial resource suggests that they were forewarned.
It seems very likely that the conspirators intended to restore the Republic and return Rome to senatorial rule. It also seems likely that the conspiracy was carefully planned and the many senior individuals were involved.