Why did the senator oppose Caesar?

Answer 1

The group of senators who opposed Caesar were the ones who thought that if he remained in power the government of the Roman republic was doomed. They could see no other way to rid themselves of his authority except by assassination.

Answer 2

There were suspicions, probably justified, that Caesar was trying to become a king. This was anathema to Republican culture. The Republic had been established in some 460 years earlier with the overthrow of the last king of Rome who was a tyrant. The Romans decided to do away with the monarchy and to establish a Republic headed by two annually elected consuls. The aim of the Republic had been not to leave power in the hands of a sole man so as to prevent the return of tyranny. The Romans also swore that they would not allow anyone to become a king ever again. In the Early Republic trying to become a king had been the worst allegation against anyone. In the 5th century BC a man was killed and his house razed to the ground because he was accused of trying to become a king and in the 4th century BC another man was executed for the same reason.

Julius Caesar had used his military power and his enormous popularity among the Roman lower classes to concentrate power in his hands. Some of his behaviour seemed to strongly suggest that he wanted to become a king. To traditional republicans this would have meant the end of the Republic and tyranny. They equated monarchy to tyranny. Therefore, they decided to kill Caesar to save the Republic. Further justification for their action was provided by an old law of the Republic which authorised anyone to kill someone who tried to become a king and exempted them from punishment.