Brutus was not justified in killing Caesar because there was no proof that he was acting as a tyrant.
Brutus admits himself that Caesar did not do anything wrong—yet. He and his confederates kill Caesar because of what they fear he would do. If Caesar had been a tyrant mistreating his people, then they might have been justified in killing him. As it was, all Brutus had was predictions of future misdeeds based on political disagreements.
In the beginning of the play, at the Feast of Luperacal, Caesar specifically refuses a crown offered to him by Mark Antony. It would seem based on these actions that he does not want to be king.
Antony offers him the crown three times, for reasons we are not sure of, and each time he refuses it. The people cheer because they love Caesar and are glad he is not positioning himself as king. To Romans there is nothing wrong with being a dictator, which was a temporary rank appointed by the senate, but to be a king was terrible. Caesar specifically said that he did not want to be king.
Antony comments on the crown incident in his speech following Caesar’s death. He uses it as an example of how Caesar was not as ambitious as Brutus has been trying to paint him in order to justify killing him.
You all did see that on the Lupercal
I thrice presented him a kingly crown,
Which he did thrice refuse: was this ambition?
Yet Brutus says he was ambitious;
And, sure, he is an honourable man. (Act 3, Scene 2)
Antony says at the beginning of the speech that he has “come to bury Caesar, not to praise him” but he goes out of his way to impress upon the people that Caesar loved them and did not want to be king. Antony reminds them that although Brutus says Caesar is ambitious, they do not see evidence of it.
Brutus himself points out that Caesar has done nothing wrong in the soliloquy he gives before the conspirators meet at the beginning of the second act. In it, he explains why he feels that Caesar should die. It is a preemptive move. Caesar is not powerful now, but will be in the future. He is not dangerous now, but will be. Brutus makes all of these justifications, but even he points out that as well as he knows Caesar, he has no reason to kill him yet.
It must be by his death: and for my part,
I know no personal cause to spurn at him,
But for the general. He would be crown'd:
How that might change his nature, there's the question.
It is the bright day that brings forth the adder;
And that craves wary walking. (Act 2, Scene 1)
In other words, Brutus knows that Caesar is fine now, but he fears what will happen when he is crowned King Caesar. Brutus seems to think this is an inevitable thing. At that point, even though Brutus has no cause now to hate Caesar, he fears that Caesar will become destructive. He comments that being crowned might change Caesar’s nature, and make him like the snake you have to watch out for. For this reason, they must kill him in his shell while he is still a baby snakeling not yet hatched.
Brutus explains to the people in his speech preceding Antony, after Caesar was killed, that he and his people were liberators. He explains that to live under Caesar would be to live a slave. Again, he explains that Caesar was ambitious, and it was for this reason and this reason alone that they killed him.
As Caesar loved me, I weep for him;
as he was fortunate, I rejoice at it; as he was
valiant, I honour him: but, as he was ambitious, I
slew him. There is tears for his love; joy for his
fortune; honour for his valour; and death for his
ambition. (Act 3, Scene 2)
Unfortunately for Brutus, the people do not see it that way. Swayed by Antony’s moving speech, in which he calls Brutus and his followers hypocrites, the people decide that Brutus and the conspirators were wrong in killing Caesar and condemn them as murderers. For this reason, Brutus and the others must flee before the crowd tears them limb from limb once it becomes an angry mob.
Brutus really did believe that he was acting honorably, despite Antony’s sarcasm. For whatever reason the conspiracy was founded, as its leader Brutus made an effort to keep it on the level. He believed that what he was doing was right, and really did consider himself and the others liberators. Basically, Brutus believed his own persuasive rhetoric. The people of Rome did not. Julius Caesar was not seen by them as a tyrant. He was a beloved leader.