Brutus offers the audience insight into what motivates him to join the conspirators in assassinating Julius Caesar in act 2, scene 1. When Lucius exits, Brutus contemplates murdering Caesar and begins by mentioning that he has no personal grievances against Caesar. Brutus then wonders how attaining ultimate authority might alter Caesar's behavior. Brutus also questions Caesar's ambition and admits that he has never witnessed Caesar's emotions get the better of him. Brutus then reveals the reason he will join the conspirators by saying,
Fashion it thus: that what he is, augmented, Would run to these and these extremities. And therefore think him as a serpent’s egg—Which, hatched, would as his kind grow mischievous—And kill him in the shell (Shakespeare, 2.1.30-34).
Essentially, Brutus is saying that in order to prevent Caesar from ruling Rome as a tyrant, he must kill Caesar before Caesar gets the opportunity to rule as a tyrant and allows his ambition to negatively affect his behavior.
Brutus also justifies assassinating Caesar just before Mark Antony gives his funeral oration. Brutus tells the masses that in killing Caesar, he was doing the Roman populace a favor by preventing the rise of a future tyranny. Brutus reveals his honorable intentions by telling the crowd,
With this I depart: that, as I slew my best lover for the good of Rome, I have the same dagger for myself when it shall please my country to need my death (Shakespeare, 3.2.42-44).