Cassius is always a lesser figure than Brutus in Julius Caesar. Brutus is a tragic figure, a person of honor who truly cares for Rome and who is defeated while trying to do the right thing. Cassius, on the other hand, is a great manipulator, and so his relationship to Brutus is a manipulative one.
While Brutus puts the interests of Rome first, Cassius is self-interested. Cassius wants to assassinate Caesar because it personally annoys him that Caesar is so popular and rising above the rest of the senators. He does not want to bow down to a person who was once his equal, which he fears is about to happen.
Cassius knows, however, that killing Caesar is risky, which is why it is so important to have Brutus on board with the plot. Brutus has a sterling reputation and commands a great deal of respect. If he participates in the murder, he will lend legitimacy to the murderers.
Cassius, a cold-blooded (and somewhat cowardly) politician, works on Brutus' sense of worry that Caesar gaining too much...
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