Even though Brutus does not dislike Caesar as a person, he believes that Caesar, who was already beginning to be a tyrannical leader, would become more so as he gained power. While Brutus is not a violent man and does not want to commit murder, when Cassius presents the plan of the conspirators to Brutus, he sees no other option for his attempt to save Rome from the tyranny that seems to be in their future. Therefore, he allows himself to be persuaded by Cassius to join the men planning to assassinate Caesar. Brutus does not deliver the first attack, but it can be argued that he delivered the murderous strike: it was at the point when Caesar realized Brutus had joined the conspirators that he stopped fighting back and conceded to death.
