Brutus can be accounted as a tragic hero because he is unfailingly presented as a noble, upright, virtuous man who is, however, led into the tragic act of betraying a friend; and he suffers both internal and external conflict as a result.
Shakespearean tragic heroes, following the model laid down by Aristotle, generally are characters who are upstanding figures, well-spoken of by everybody, but who are let down by one major flaw. Brutus fits into this template. His flaw is his idealism – although it might seem odd to label idealism as a flaw, especially when compared to the deadly ambition of a Macbeth or the all-consuming jealousy of an Othello. However, it is undeniable that Brutus’s idealism leads to a fatal naivety on his part. He first is naïve enough to think that his political idealism can offset all personal concerns in turning against Caesar, a close friend.
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 crowned.
How that might change his nature, there’s the question. (II.i.10-13)
Brutus has to concede that Caesar is not actually an oppressive tyrant and thus tries to justify his assassination purely on hypothetical grounds: that he may yet become one. This causes him considerable mental and emotional turmoil; he likens his state to that of a 'kingdom' suffering a 'insurrection' (II.i.68-69). His actions engender civil war in Rome, but long before this he is already beset by inner conflict.
It is true that Cassius’s machinations also propel Brutus towards killing Caesar, but, tellingly, he remarks that he is ‘with himself at war’ (I.ii.46) before Cassius tries to convince him to join the conspiracy. As a long-standing friend, Cassius know him well and simply works on bringing to the surface the tensions that already exist within him.
After the assassination, Brutus’s naivete misleads him into believing that he can win everybody around to his cause. Instead, events spiral completely out of his control; Rome is engulfed in civil war, and he and Cassius are defeated and take their own lives. The idealistic vision which he had for the state of Rome, as an enduring republic, is completely destroyed. Ultimately, his action in killing Caesar in order to safeguard the republic has the wholly ironic result of helping to establish Octavius, Caesar’s grand-nephew, as the first emperor of Rome. The tragedy of Brutus is not just his act of betrayal, his defeat and death, but the death of his ideals.