Cassius, weakened by the loss of those troops and by deserters, is losing his half of the battle to Antony. He sends Titinius on horseback to see whether his camp is being burned, and sends Pindarus to mount a hill and watch Titinius. Pindarus sees Titinius overtaken by other riders. Cassius, thinking the battle is lost, orders Pindarus to kill him. Pindarus kills Cassius with the same sword Cassius used to stab Caesar. Pindarus then flees Rome forever.
While suicide is not out of line with Cassius's beliefs, faith in omens is, and Cassius's hasty assessment of the battle's outcome is apparently influenced by his interpretation of the carrion birds that perched on his standards.
Titinius enters with Messala. It turns out the other horsemen were allies bringing news of Brutus's victory over Octavius. Titinius is bearing a wreath of victory from Brutus to Cassius. They notice Cassius's body, and Titinius sends Messala to tell Brutus. Alone with Cassius's body, Titinius draws out Cassius's sword and kills himself out of grief.
The omen Cassius saw was paradoxical. It influenced him to believe the battle was lost when it wasn't, so he killed himself, which causes his forces to lose the battle. It wouldn't have come true if he hadn't believed it.
Brutus enters with Messala, Young Cato, Strato, Volumnius, Lucillius, Labio and Flavius. On seeing the bodies, Brutus remarks that Caesar's Ghost is hunting them all down. He gives orders for Cassius's body to be taken to Thasos, adding that the battle is a standstill so far, and will soon be joined again.
Brutus's comment reflects not so much a fear of Caesar's Ghost as a growing belief that their deaths are deserved. For Brutus, situations that progress from wrong action can never be righted.