All of the drama of Cassius's death takes place in Act 5 scene 3. In this section of the play, Cassius is standing on a hill watching the battle. The battle is not going as planned and Brutus and his troops seem to be losing.
Pindarus comes to tell Cassius that he should flee, but Cassius refuses to do so. He then asks Titinius, his best friend, to see what is happening. So Titinius rides out. Cassius finds out that Titinius is surrounded by men (Pindarus is giving the play by play.) Cassius believes that his friend is captured. So he tells Pindarus to watch no longer. Instead, he asks him to kill him. Cassius’s last words are that Caesar has now been revenged.
"Caesar, thou art revenged, Even with the sword that kill'd thee."
All of these actions so far were based on wrong information. Titinius is not dead, but when Titinius finds out all that has happened, he mourns, and then he takes his own life.
"Why didst thou send me forth, brave Cassius? Did I not meet thy friends? and did not they Put on my brows this wreath of victory, And bid me give it thee? Didst thou not hear their shouts? Alas, thou hast misconstrued every thing! But, hold thee, take this garland on thy brow; Thy Brutus bid me give it thee, and I Will do his bidding. Brutus, come apace, And see how I regarded Caius Cassius. By your leave, gods:--this is a Roman's part Come, Cassius' sword, and find Titinius' heart."
It all ends tragically. And in a sense, Cassius is redeemed.
Cassius thinks that Titinius has been overtaken and that his side has lost the war. Ashamed that his friend has been killed and distraught over his supposed defeat, he has his servant Pindarus run him through with his sword. In actuality, Titinius was being greeted by his allies and was making his way to tell Cassius of Brutus' victory over Octavius.