Answer:
Brutus was angry with Cassius in act iv, scene iii because Cassius was taking bribes.
Step-by-step explanation:
Through the story, Brutus sees many things on Cassius that little by little make him feel disappointed. One of them is the discovery of the fact that Cassius was taking bribes which makes Brutus doubt Cassius honor, as he had already doubted before with the topic of Caesar's assassination. Brutus noticed the personal benefit and selfishness of Cassius.