How did Gaius Cassius Hemina die?

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.