While the reader has been led to believe in Brutus' strength of nobility, there is a touch of weakness in the self-delusion he must create before he can join the conspirators: Brutus feels that murder is wrong and so must find a way to justify his actions. It's not for personal reasons that he will do it, but for the general; that is, for the good of the people of Rome. He generalizes about the effects of power and ambition and anticipates the damage that Caesar will do when he gains the crown. He has to admit, however, that Caesar has not yet committed any of these wrongs. Brutus has to convince himself to kill Caesar before he has the opportunity to achieve his ambition; that is, he will "kill him in the shell." The final element of his persuasion comes from an outside source. He responds to the call of the people without knowing that the call is false. The letters that Cassius has penned have been discovered in Brutus' closet; he reads them and is persuaded by them under the same harsh and distorting "exhalations of the air" that light the conspirators' way to Brutus' doorstep. By that light, one can see that Brutus is as tainted as any of the other conspirators.
Brutus, although he has decided to be one of the conspirators, knows that what they plan is wrong. "O Conspiracy, / Sham'st thou to show thy dang'rous brow by night, / When evils are most free?" (emphasis added). But being a man of his word, he is committed to the plan. After a brief, whispered discussion with Cassius, Brutus takes on the leadership of the group, and when Cassius calls on the group to swear to continue as they have planned, Brutus stops them, and begins by a sort of negative persuasion to fix their resolve and establish himself as leader. "No, not an oath!" he says. If their motives are not strong enough, an oath will not help them to accomplish the deed. Only cowards and deceivers would swear, and to swear would be to taint what they do. This is how Brutus convinces his men. He creates a void, takes away what Cassius says, and then fills it with his own voice. By stripping away the words of an oath and by replacing that oath with images of valiant Romans, their very blood carrying strength, nobility, and constancy, Brutus inspires his men and establishes himself as their leader. Caesar, therefore, is not alone in his ambition.
