Brutus's soliloquy in Act II, Scene 1 reveals his thoughts on the possibility of assassinating Caesar. He is torn, as Caesar is his friend, and is very popular with the people. But he fears that if Caesar becomes a king, not only will the Republic be destroyed in the process, but Caesar, who has not really done anything terribly wrong thus far, will do so in the future. In particular, he has not shown himself to be a tyrant. If he becomes king, though, he may well become corrupted by power, and for Brutus, that risk is not worth taking. He says that he and the other senators must
...think him as a serpent's egg
Which hatch'd would as his kind grow mischievous,
And kill him in the shell.
This is a serious decision, and a turning point in the play. As he grapples with the question of whether or not to join the plot, Brutus is revealed to be a man of principle who has the interests of Rome, rather than his own ambition, at heart.
In Act II, Scene I, Brutus is up at night ruminating about how to deal with Caesar's ambitions, which Brutus senses are to become a dictator. He resolves to kill Caesar. Though Caesar seems reasonable at this point, Brutus says, "That lowliness is young ambition’s ladder,/ Whereto the climber upward turns his face" (lines 22-23). In other words, young leaders like Caesar act humble and use this humility to ascend to power. They then reject humility in favor of arrogance. Brutus is thoughtful yet willing to take action to rid Rome of Caesar.
Brutus feels compelled to act to prevent Caesar from taking power because of the weight of his ancestry. He says that his ancestors drove Tarquin away when he wanted to be king, and Brutus feels a responsibility to continue to be the watchdog of Rome. He feels compelled to take action, and he tells the band of conspirators gathered at his door that they must do so because their own souls are suffering, as are those of the people around them. He says, "If not the face of men,/ The sufferance of our souls, the time’s abuse—/ If these be motives weak, break off betimes,/ And every man hence to his idle bed" (lines 120-123). In other words, if the suffering of the conspirators' own souls and those of the people around them are not enough reason to kill Caesar, all the conspirators should return to bed. Brutus feels that his conscience is urging him to take action, and he feels responsible for protecting those around him.
However, Brutus suffers as a result of his decision to kill Caesar. He says, "Between the acting of a dreadful thing/ And the first motion, all the interim is/ Like a phantasma or a hideous dream" (lines 65-67). He says that since deciding to take action he has suffered, as if he were living in a nightmare.
Brutus is also wary of shedding too much blood and shows restraint in his actions. He says, "Let us be sacrificers but not butchers, Caius" (line 173) and tells the band of conspirators that they should not kill Antony as Cassius suggests. Brutus feels that killing Antony and Caesar would be far too much and make the band seem too bloodthirsty. Instead, Brutus, logical and restrained, thinks that Antony will be powerless without Caesar. In this scene, Brutus shows that he is willing to take action but does so in a logical and restrained way to protect the Roman people.