In Shakespeare's play Julius Caesar, there are indications of the tyranny of Caesar as well as his epilepsy and compromised health, two factors which point to his questionable leadership. In real history, too, of which the Romans would be aware, the Senate of Rome in 49 B.C. had ordered Caesar to hand over his army to their control, but he refused; instead, he advanced upon Italy, stopping at the border to Gaul because a Roman governor was not allowed to leave his province. Nevertheless, he later crossed the Rubicon River to confront his enemies, an act that the Senate considered as treason. One by one, Caesar brutally killed his enemies. As mentioned in the play, Caesar killed Pompey. And, for the next three years, he killed more.
With these facts in mind, the audience listening to Brutus's declaration that he loved Rome more than he loved Caesar could easily have been swayed to think that the assassination of such a tyrant was necessary, especially with his history of arrogance and desire for power and eagerness to accept the crown when he parades through the streets of Rome.
Here are some actual statements from Roman statesmen that may help in deciding how an audience could agree with Brutus:
"Our tyrant deserved to die. Here was a man who wanted to be king of the Roman people and master of the whole world. Those who agree with an ambition like this must also accept the destruction of existing laws and freedoms. It is not right or fair to want to be king in a state that used to be free and ought to be free today." Cicero.
"People blame me for mourning the death of my friend. They say my country should be preferred to my friends, as if they had proved that killing him was good for the state. I did not abandon him as a friend however much I disapproved of what he was doing." Gaius Matius.