In the beginning of the play, Brutus is indecisive about the conspiracy. He knows that Caesar is gathering power, but he hasn’t seen him use it in an inappropriate way. When Cassius approaches him about joining the conspirators he doesn’t commit himself right away.
It is worth noting that Cassius appeals to his vanity right away by referring to him as honorable and noble. Brutus prizes the quality of honor. Cassius also resorts to a bit of trickery by sending Brutus letters praising him and asking him to join the conspiracy. These letters appear to have been written by different people.
Brutus does some soul searching and finally decides that Caesar must be “killed in the shell” before he becomes too powerful to stop.
Brutus feels that he cannot risk his “honor” by allowing Caesar to assume greater and greater power. An all-powerful Caesar would eclipse and possibly, in a sense, enslave Brutus (not literally, but politically). How much of Brutus’ decision is based on true honor and how much is due to his vanity is unclear. What is clear, however, is that once Brutus makes up his mind, he does not look back.