What does Brutus say would have happened if the senators wouldn't have killed Caesar?

Brutus genuinely believes that if Caesar hadn't been assassinated then he would have made himself king. The Roman republic that Brutus venerates so much was founded after the overthrow of kings. From then on, Romans were anxious to make sure that their city state would remain a republic, and they were deeply suspicious of anyone who might give the impression of wanting to become king. By the time of his murder, Caesar had become a very powerful individual in Rome; in actual fact, he was a dictator. Many people, including Brutus, thought that the next logical step was for Caesar to declare himself king, something they would never accept.
In his speech to the plebs, or common people of Rome, Brutus attempts to justify his participation in Caesar's murder. He says that if Caesar had been allowed to live, then the Roman people would've been turned into his slaves:

Had you rather Caesar were living and die all slaves, than that Caesar were dead, to live all free men? (Act III Scene ii).

If Caesar hadn't been killed, he would've destroyed the ancient traditions of republican Rome. Even if he never actually called himself a king, he would still effectively be one. The Roman people, for their part, though still formally citizens, in reality would've been little better than slaves.