The letter states that this person does not like caesar. Brutus thinks to himself that if there are more than just the conspirators and him that do not want Caesar to be crowned than it should be done than. Even though the letter was fake and from Cassius to convince Brutus to join the conspiracy. It worked to convince Brutus even more and that is another reason Brutus joined the conspiracy. Mostly in act two scene one does the conspirators come visit Brutus to convince him but throughout the story they do. If Cassius would keep bothering Brutus and reminding him of the decision he has to make he probably would eventually cave in. When someone bothers another person all the time like a pest the person almost gets sick of hearing it that they just say yes. So this might have been a factor of why Brutus joined the conspiracy.
Does Brutus justify his reasons for joining the conspiracy? Brutus does have reasons for killing his friend and he does justify them. Brutus does not dislike Caesar he just is doing what is best for the people and him. And what is best for the people and Brutus is for Caesar to be killed before he accepts the crown. This is very relevant because it is a big decision that our time might go