He is willing to kill Caesar because Cassius convinced him that this is for the people of Rome and Brutus believes him. So Brutus thinks that the conspiracy is killing him for Rome but really they are killing him because they think he will become too powerful
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Cassius also wanted to kill Marc Antony, Caesar's closest ally.
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Mark Antony - Senator and associate of Ceasar
Cassius convinced Brutus that it was right to kill Caesar and Brutus plots to kill him.
Marc Antony