Brutus decides to join the conspiracy against Caesar after his realization to the fact that the Roman Republican government was in great danger. Caesar had himself declared himself dictator for life. (the ancient Roman position of dictator was defined as a necessary governmeantal position in times of crisis, not to be confused with the modern definition of dictator) However, Caesar's power grew ever stronger and Brutus saw this as tyrannical.The Senate to which Brutus was a member became nothing more than a puppet on the hand of Caesar.The love Brutus had towards the Republican principle and the sense of duty to secure that principle ultimately forces him to choose between a man he greatly admired and the political philosophy which was to serve the greater good. In his mind Brutus was doing what was "noble" and morally right, in this manner the assassination of Caesar had moral and political justification.
In deciding whether to join the conspiracy, Brutus must choose between his friendship with (and admiration of) Caesar and his public responsibility to prevent Caesar’s alleged ambition to undermine the Roman Republic. Cassius has convinced Brutus that Caesar plans to install himself as monarch. A monarchy would rob the Romans of their long-held liberties.
In his soliloquy in Act II, Scene i, Brutus resolves his inner conflict. He states that he will join the conspiracy because of Caesar’s ambition to be crowned:
It must be by his death: and for my part
I know no personal cause to spurn at him,
But for the general good. He would be crown’d:
How that might change his nature. . . .
. . . . . . . . . . . . . . . . . . . . . . . .
And therefore think him as a serpent’s egg
Which, hatch’d, would, as his kind, grow mischievous,
And kill him in the shell.
After Caesar’s assassination, Brutus explains his actions to the plebeians (average citizens) in the Forum (Act III, Scene ii):
. . . If then that friend demand
why Brutus rose against Caesar, this is my answer:
Not that I loved Caesar less, but that I loved
Rome more. Had you rather Caesar were living and
die all slaves, than that Caesar were dead, to live
all free men? As Caesar loved me, I weep for him;
as he was fortunate, I rejoice at it; as he was
valiant, I honor him: but, as he was ambitious, I