In Shakespeare's play, Julius Caesar, Cassius and Brutus have shown different motivations in killing Caesar. They have argued with one another while each tries to follow his own vision.
First, the relationship between Brutus and Cassius is based on a lie. Brutus was concerned that if Caesar became king, it would change him which would harm Rome. Brutus, with all his faults, put the good of Rome before all else.
Cassius, on the other hand, approaches Brutus to join the conspirators in assassinating Caesar. Brutus is not sure, but Cassius plans to falsify papers that will prove Caesar has committed crimes against the state of Rome. Ultimately, Brutus joins. Cassius, however, does not share the noble reasoning that governs Brutus' actions: Cassius hates Caesar because he once saved Caesar's life and believes he has not been rewarded enough—that he is treated the same as every other citizen of Rome. His reasons for killing Caesar are completely selfish.
It is, therefore, not surprising that the two would fight. At one point, the two fight because Cassius believes that Brutus has mistakenly judged one of Cassius' men for "taking bribes." What sits at the center of their fighting, though, is the level of "love and honesty" the men have for each other—at least from Cassius' perspective. Even as they make plans to got into battle, they cannot agree upon their strategy.
Cassius is the one behind the plot to kill Caesar. He is an excellent judge of men, their strengths, weaknesses, etc. However his is not completely honest with Brutus, and he also fails to take a stand against Brutus when he does not agree with him, something he would probably not do with another man. (Cassius might resent this.) Cassius tricks Brutus into joining the plot, though he cannot prove any of the criticisms he has of Caesar. The best Cassius can do is criticize Caesar's physical shortcomings; he cannot admit that Caesar's leadership and superior military strategies have allowed him to be so successful as Rome's leader. Cassius agrees with some of Brutus' decisions, though his heart does not agree. For instance, Cassius wants to kill Mark Antony and Caesar, but Brutus disagrees, wanting them to be guilty only of "sacrifice" with Caesar, but not "butchery" which Brutus believes would be the case if they kill Antony.
Later the two men have a terrible fight, where Cassius accuses Brutus of no longer loving him; Brutus says he does not like Cassius' faults. Cassius says that if Brutus no longer loves him, he would prefer Brutus to kill him there. Cassius says:
There is my dagger,
And here my naked breast; within, a heart (110)
Dearer than Pluto's mine, richer than gold.
If that thou best a Roman, take it forth;
I, that denied thee gold, will give my heart.
Strike, as thou didst at Caesar, for I know,
When thou didst hate him worst, thou lovedst him better (115)
Than ever thou lovedst Cassius.
In some ways Cassius seems rather pitiful in his approach to Brutus; Brutus comments that Cassius too quickly becomes angry and then becomes calm again. The men brush aside their differences and they are of one mind again. Brutus then admits that his wife, Portia, has killed herself.
Perhaps the murder of Caesar begins to take its toll. Cassius will never truly be able to battle his sense of insecurity, as he felt with Caesar, and Brutus is an honorable man who killed the leader he loved. Now his wife is dead. It is no wonder the two have grown apart.