Because Cassius resents the fact that the Roman people have looking up to Caesar as if he was a god, and he did not like that.
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There was not a Gaius Cassius Hemina. There was a Lucius Cassius Hemina who was a historian and wrote the first annals in around 146 BC. If you are thinking of the Gaius Cassius who was an instigator in the plot against Julius Caesar, his name was Gaius Cassius Longinus. He ordered his freedman, Pindarus, to kill him when he lost the battle of Philippi.
cicero
Pindarus
Killing Brutus and Cassius.
No one knows exactly how the assassination was done in history, but in the play written by William Shakespeare, Brutus was able to maneuver Cassius in such a way as to take control of the group. Cassius didn't intend to let it happen, but Brutus did plan the strategy and make most of the important decisions.