He realizes that he is going to die because Brutus gave the orders too soon and Antony's men have the advantage, so he feels like suicide is his best option.
He sees that his best friend, Titinius, has been captured and feels as though he can no longer go on living since Titinius' death is on his hands; therefore, killing himself is the only way he sees as an option take responsibility .
He decides to kill himself with the same sword he stabbed Caesar proving to be ironic since Caesar's assassination was the whole reason for the blood that had been spilled in the first place.
Cassius realizes that all hope is lost for him to ascend the throne to rule over Rome. He has dug himself into such a deep hole that the only way out is through killing himself.
Cassius decides, after thinking about his actions, that he and the other conspirators were wrong in killing Caesar. The only form of retribution appropriate for his actions is self murder.