Cassius... friends... I simply cannot betray Caesar. I refuse to engage in this mutiny, so go on without me.
Traitor! You were never worth our time! Perish now, so you will not meddle in our business later!
On the hour of the planned assassination of Julius Caesar, Brutus tells Cassius and the other conspirators that he will not partake in the attack (III, i).
With no friends to help me, I believe I have already lost this fight... But I cannot admit defeat. Not yet. I need to end this war as quickly as possible to have a chance at winning. I shall bring my army to Philippi tomorrow morning, so we can catch our enemies by surprise.
Thinking that Brutus had betrayed the conspirators, Cassius quickly stabs and kills Brutus in anger (III, i).
Ha, who comes here? Brutus? Is thee really here? Speak!
I will not listen to you! You have betrayed me, Brutus, and I am not forgiving. I am going to Philippi, and I will win this battle on my own!
Aye, it is Brutus. Cassius, do not go to Philippi, for they are prepared and you will not win the fight. You must leave Rome! Flee now! It is the only way you can live.
Wanting to not delay any more time, the conspirators quickly move along with their plan and kill Caesar. The conspirators then announce the death of tyranny in Rome before confronting Antony and his servant (III, ii).
No! I will go to Philippi, and that is my final decision. You cannot deceive me again Brutus. Leave at once!
Cassius, I have forgiven you for taking my life, as it was not thee fault... It was mine. You can trust me Cassius... Do not go to Philippi.
The other conspirators flee Rome unnoticed, leaving Cassius to fight the triumvirs alone (along with his army). Cassius is forced to make his own decisions, but he struggles with making them. Alone and worried, Cassius eventually decides to go to Philippi to fight the triumvirs and end the war quickly (IV, i).
Cassius is greeted by the ghost of Brutus, who tells him to leave Rome and to not fight. Cassius thinks that Brutus is trying to get his revenge, and so he doesn't listen (IV, ii).
Brutus explains his rationale for helping Cassius, but Cassius refuses to believe what Brutus says. The ghost of Brutus leaves once Cassius asks him to (IV, ii).