Cassius wants to organize a group of disaffected nobles to assassinate Caesar and feels that he needs Brutus because of Brutus's ancestry and reputation for integrity. Once Brutus agrees to join the others he takes over the leadership and becomes more and more despotic. Cassius can't control him, even though Cassius organized the conspiracy and has more practical wisdom and cunning than Brutus.
When Cassius wants Antony killed along with Caesar, Brutus overrules him. When Cassius warns against permitting Antony to speak at Caesar's funeral, Brutus smugly overrules him, thinking that his own oration will be more effective than anything Antony can say. Finally when Brutus and Cassius meet in Act 4, Scene 2, they have a heated argument during which Cassius actually threatens to kill Brutus. But here Brutus asserts his moral superiority and forces Cassius to back down. At one point Brutus says, "Away, slight man." And then:
Fret till your proud heart break.
Go show your slaves how choleric you are,
And make your bondmen tremble. Must I budge?
Must I observe you? Must I stand and crouch
Under your testy humor? By the gods,
You shall digest the venom of your spleen,
Though it do split you. From this day forth
I'll use you for my mirth, yea for my laughter,
When you are waspish.
Brutus is asserting his complete domination of the partnership. His leadership has gone to his head. He is becoming as egotistical as Julius Caesar. Brutus can do without Cassius, but Cassius knows he cannot do without Brutus. After this showdown Brutus insists on making all the decisions. When Cassius is reluctant to fight Antony and Octavius at Philippi, Brutus overrules him, and they are defeated in the battle.