There are many reasons for both Brutus and Cassius to lead the assassination plot and to lead in the Civil War that follows Julius Caesar's assassination.
Both Brutus and Cassius have their own strengths and weaknesses.
Brutus has many strengths, which are shown throughout the play. Out of all of them, the one that characterises him the most is that he is considered a noble man and an idealist. Brutus is Caesar's great friend who joins the conspiracy because his love for Rome is greater than his love for his friend. There is no doubt about Caesar's love for Brutus, although there is little time in the play for this to be demonstrated. Caesar's love for Brutus is clearly shown through his murder. Caesar defends himself against the conspirators, running in all directions with his body. But when he sees Brutus with his sword drawn in his hand, we hear the famous cry, "Et tu, Brute" when Caesar realises that even his good friend is one of the conspirators. With this, he pulls his gown over his head and makes no more resistance.
Throughout the play, Brutus is always aware of Caesar's love and feels extremely ungrateful in return. When Brutus has been defeated and runs onto his own sword to avoid being captured, he seems to welcome his death as though it were almost a punishment for his offence in killing Caesar: "Caesar now be still; /I kill"d not thee with half so good a will" (Act 5, Scene 5, Lines 51-2). As Brutus understands them, the needs of Rome are more important than the demands of friendship. In his speech to the Roman people, Brutus offers justification for his act: "not that I loved Caesar less, but that I loved Rome more" (Act 3, Scene 2, Lines 20-3).
Antony remarks that Brutus was the only one of the group who did not kill Caesar because of envy and personal grievance. Brutus acted in what he thought were the best interests of Rome. Through the play, we begin to form a good opinion of Brutus from what the other characters say of him.