Shakespeare develops Brutus as a tragic hero. He is a good man whose fatal character flaw brings about his own destruction. The flaw in Brutus's character is that he is the complete idealist. He is often unaware of the realities of political life in Rome and of the other characters' less than honorable motives.
Brutus joins the conspiracy because he believes Caesar is a danger to freedom in Rome; he views Caesar as a potential tyrant. He is encouraged in this belief by Cassius, who plays upon Brutus's innate sense of loyalty to Rome and her democratic traditions.
Another basic characteristic of Brutus is that he is trusting to the point of being naive. He trusts Cassius, who tricks him into joining the conspiracy for Cassius's own purposes. Brutus trusts that all the other conspirators are as honorable as he. When Antony asks to speak at Caesar's funeral, Brutus trusts that Antony will abide by their agreement that Antony will say or do nothing to incite the crowd against the conspirators.
Brutus is above all an honorable man. He remains faithful to his principles even though in doing so he commits a terrible act in murdering Caesar, his friend. Ultimately, Brutus chooses to die with honor rather than be captured on the field of battle. He takes his own life by running on the sword used to assassinate Julius Caesar.
Shakespeare's final assessment of Brutus is expressed in the words of Antony at the play's conclusion: "This was the noblest Roman of them all."