Final answer:
Brutus is angry with Cassius for allegedly taking bribes, which threatens the integrity and the cause of their campaign against Caesar's rule.
Step-by-step explanation:
Brutus is outraged with Cassius because he has heard rumors that Cassius is accepting bribes. This deeply upsets Brutus as accepting bribes is a corrupt act and it goes against the moral code and the cause for which they have taken a stand.
Their mission was to restore the Roman Republic by ending Julius Caesar's rule, and any act of corruption, such as taking bribes, would undermine their moral high ground and potentially lead to the failure of their cause.