Many critics argue that Brutus is the tragic hero of "Julius Caesar," and, as such his tragic mistake is, perhaps, that he does not know himself fully. For, he feels so strongly that he is a man of principle that he does not recognize more selfish motives in others such as Casca and Cassius. In Act II in his soliloquy of Scene I, Brutus considers the possibility of Caesar's becoming a danger to Rome, reasoning that if Caesar be crowned king, he will become too selfish in his desires:
...He would be crown'd/How that might change his nature, there's the question....The abuse of greatness is when it disjoins/Remorse from power....But 'tis a common proof that lowliness is young ambition's ladder,/....But when he once attains the utmost round,/He then unto the ladder turns his back,/Looks in the clouds scorning the base degrees/By which he did ascend....(II,I,12-23)
So, in loyalty to principle, Brutus reasons that the only way to save Rome from tyranny is to kill Caesar:
It must be by his death: and for my part,/I know no cause to spurn at him,/But for the general....(II,i,10-11)
Of course, to Julius Caesar himself, the assassination is a terrible act of disloyalty, and when Brutus stabs Caesar, Caesar cries in disbelief, "Et tu Brute?" since this act is perceived by Caesar as a betrayal of their friendship. Ironically, it is also an act of betrayal of Brutus to himself as he has allowed himself to be duped by Cassius, "the lean, hungry man" whom Caesar points out to Marc Antony as desiring power. Tragically, Brutus assumes that Cassius's intentions are as noble as his own. And, because of his tragic mistake, his humarita as Aristotle defines this mistake in his Poetics, Brutus is ironically a traitor in his loyalty to principle. For, the subsequent triumvirate of Antony, Octavius, and Lepidus becomes much more dictatorial than Julius Caesar, and the man of principle, Brutus, "the noblest Roman of all" as Antony eulogizes him, dies in failure.