There is no indication in Shakespeare's play that Brutus had any intention of killing his good friend Julius Caesar, a man who had always treated him with extreme kindness and generosity. Brutus does not seem capable of initiating the kind of conspiracy that Cassius conceives and organizes. The assassination has to be carefully planned and orchestrated. Caesar is to be surrounded by petitioners who are not only confining him in their midst but are also keeping everyone else out. Antony has to be drawn away from Caesar's side on some pretext so that Caesar will have no one to protect him. Brutus is not the kind of person who would think of such subterfuge and chicanery. But Cassius certainly is.
Brutus tells Cassius in Act 1, Scene 2 that he does not like the way Caesar is so blatantly appealing to the masses in order to assume supreme power and kingship. At one point he asks:
What means this shouting? I do fear the people
Choose Caesar for their king.
Cassius seizes on that and asks:
Ay, do you fear it?
Then must I think you would not have it so.
To which Brutus responds:
I would not, Cassius, yet I love him well.
Brutus doesn't like the idea of Caesar becoming king, but so far he has done nothing about it except to distance himself from Caesar a little. When he asks Casca what happened to make the crowd shout so often, Casca replies:
Why, you were with him, were you not?
Casca naturally assumes that Brutus would have been with Caesar because the two men are such good friends and constant companions. But this bit of dialogue shows that Brutus is distancing himself from Caesar because he does not want to appear to be sanctioning Caesar's political maneuvering. It would seem that Brutus, characteristically, is simply withdrawing from the situation and not taking sides. Brutus is a loner, a scholar, an introvert, a thinker. He is not the type of man who would dream up an assassination plot. In fact, Brutus is a lot like Shakespeare's Hamlet. Hamlet does nothing about avenging his father's murder, and Brutus does nothing about preventing Caesar from taking over total Roman rule. Brutus has to be goaded into participating in a conspiracy which he had nothing to do with forming. Cassius sees in Act 1, Scene 2 that Brutus can be persuaded to join him in his dangerous and complicated plot, but Brutus will need a lot more persuading. Cassius decides on a further tactic:
I will this night,
In several hands, in at his windows throw,
As if they came from several citizens,
Writings, all tending to the great opinion
That Rome holds of his name, wherein obscurely
Caesar's ambition shall be glanced at.
And after this let Caesar seat him sure;
For we will shake him, or worse days endure.
Cassius is saying what, according to Plutarch, actually happened in history. These forged letter convince Brutus that the citizens are all hoping and praying he will take the leadership in whatever is necessary to prevent Caesar from becoming a king. He is made to feel that, because of his distinguished ancestry, it is his obligation to take the same kind of action his ancestors took to rid Rome of the Tarquins. Unlike Cassius, Brutus cannot be moved by selfish motives. He is moved by pride in his ancestry and his concern for the good of Rome.
So without Cassius' persuasion and cunning machinations, it is unlikely that Brutus would have taken any independent action against his friend Caesar. Cassius himself can see that doing so would be contrary to Brutus' interests. After Brutus leaves in Act 1, Scene 2, Cassius shows his true colors when he says to himself:
Caesar doth bear me hard, but he loves Brutus.
If I were Brutus now and he were Cassius,
He should not humor me.