Why is Brutus's speech at the beginning of Act II considered sincere?

Throughout the play, many characters come to a conclusion that Caesar must be killed, Brutus included.  However, Brutus is the only conspirator in the play who acts entirely out of sincerity.  While the other conspirators seek to gain power by eliminating Caesar, Brutus honestly believes that Caesar's climb in authority would be bad for Rome.  In this scene, Brutus mulls over his dilemna, weighing out the facts for himself.  While he is still loyal to Caesar, he also fears that Caesar would become corrupt by a gain in power or status. 

After Brutus's servant enters and delivers the letter, Brutus finds that his first impressions were solid.  He believes that the letter details the wishes of the Romans to be rid of Caesar; since Brutus always tries to act in the best interests of Rome, he determines that he must follow through with the conspiracy plot.  Once he measures the pros and cons of the situation, Brutus comes to a decision that Caesar must die, for the good of Rome.