In what is known as the seduction scene of Shakespeare's "Julius Caesar," Brutus has been convinced in Act I, Scene 2, by his friend Cassius that Caesar is
like a Colossus, and we petty men/Walk under his huge legs and peepabout/To find ourselves dishonorable graves...When could they say [till now] that talked of Rome,/That her wide walks encompassed but one man?
So convincing is the envious Cassius that Caesar wishes complete power that Brutus promises to consider what his friend has said. Later, in his soliloquy in the opening scene of Act II, Brutus convinces himself that Caesar must die, not because he has abused power, but because he may do so in the future. Indeed, Brutus is so seduced by the words of Cassius, believing that he himself perceives a reason for preventing Caesar to continue to lead Rome when, in actuality, he merely echoes Cassius's thoughts, saying that he must be stopped, in case he may desire complete power.
...But 'tis a common proof/That lowliness is young ambition's ladder./Whereto the climber upward turns his face;/But when he once attains the upmost round,/He then unto the ladder turns his back,/Looks in the clouds, corning the base degrees/By which he did ascend. So Caesar may;/Then lest he may, prevent.
The irony here, of course, is that when Brutus should not have listened to Cassius, he does; when he should listen to Cassius in the later acts as they go to battle in Philippi, Brutus does not. Perhaps, Brutus wishes to think for himself after having been so swayed by Cassius that he killed his friend.
Brutus believes it is necessary for Caesar to be killed because he believes that Rome will be better off if it is run by the senators, as opposed to a single ruler. He fears that not only will Caesar become a dictator of Rome, but also that that is what Caesar desires. In order to preserve the Republic, Brutus sees no other solution than to kill Caesar, even though they are good friends and have respect for one another. Of all of the conspirators, Brutus seems to be the only one looking out for the good of Rome; the other men seem to have personal gripes against the new ruler.
Brutus believes that Julius Caesar needs to be killed to protect the Roman political system. He thinks that Caesar wants to have too much power for himself and that this would be bad for Rome.
Up until this point, Rome had been a republic. But Caesar has been getting more power for himself and seems to be on the verge (in Brutus's eyes) of turning Rome into a monarchy. He believes that Caesar has become too popular and that the people will make Caesar a dictator because they like him so much.
Brutus feels that Caesar must be killed so that the Roman Republic might be able to continue.
Brutus has totally been brainwashed by Cassius whose motives appear to be nothing more envy and jelousy. Lines 121-3 Act 1 scene 2:
And this man
Is now become a god, and Cassius is
A wretched creature, and must bend his body
If Caesar carelessly but nod on him.
We know that Brutus is gradually getting convinced by Cassius when in Line 140 by the in his comment when he reports about ' some new honors that are heape'd on Caesar'.
The final straw is the the soliloquy about the haves and have-nots and how the prospect of dictatorship in Rome is not acceptable to Cassius and eventually Brutus.
Like Colossus, and we petty men
Walk under his huge legs....