What event leads to Cassius's death in Julius Caesar?
Cassius sends Titinius to see if the men in his tents are allies or the enemy. His slave, Pindarus, goes to report what happens as Titinius does this. Pindarus reports back to Cassius that he saw the men surround Titinius, shouting, and that Titinius was taken off the horse. Cassius makes his own assumptions about the report he receives and believes that his best friend has been captured by the enemy. Cassius is shamed by his friend's capture, and bargains with Pindarus. He will give Pindarus his freedom in exchange for Pindarus stabbing him to death.
Pindarus eventually agrees, and Cassius dies. Unfortunately, Cassius had jumped to the wrong conclusion. The men were Brutus's and they were surrounding Titinius with the news of Brutus's victory over Octavius. They were shouting for joy.
Upon discovering Cassius's body, Titinius kills himself out of loyalty to his friend.
Cassius understands that the war is about over, and they will lose the fight. He sends both Titinius and Pindarus out to see what his happening, realizing his “life is run his compass” (5.3.26). Indeed, such is the case, for Pindarus reports that Titinius has been surrounded and taken by Antony’s men. When Pindarus returns, Cassius asks him to stab him with the sword “that ran through Caesar’s bowels,” saying “Caesar, thou art revenged / Even with the sword that killed thee” (5.3 50-51). In many ways, Cassius was the instigator behind the assassination; he brought Brutus into the conspiracy, thereby giving it legitimacy. Here he punishes himself, or more accurately, allows Caesar (figuratively) to punish him for his actions.