Cassius kills himself in despair after he hears erroneous information about his friend Titinius being captured at the Battle of Philippi. Titinius is not captured, and when he returns to camp and sees the body of his friend Cassius, he too commits suicide.
Cassius was the primary author of the plot to assassinate Julius Caesar. The traditional reading of Shakespeare's play holds that he plotted against Caesar out of envy and ambition. His ally Brutus chose to join the conspiracy out of honor and patriotism toward Rome and to save it from what he thought was Caesar's ambition. Later in the play, Cassius and Brutus wage war against the alliance of Mark Antony and Octavian. They both die by their own hands at Philippi.