Brutus wants what is best for Rome, and he kills Caesar out of civic responsibility, like he has too like its his job and he knows that if no one kills Caesar, Caesar will become tyrant of Rome and the Republic will be over. Brutus cares for Rome greatly and knows what could happen if Caesar wasn't killed. All of the other characters in the play don't act as honorably as they should and are only interested in how Caesar's death will benefit themselves and not Rome.