Another reason why Brutus is not seen as the hero of the play is because his actions have disastrous consequences for Rome. His assassination of Caesar leads to a civil war, which results in the deaths of thousands of people and the destruction of the Roman Republic. Brutus's good intentions may have been honorable, but his actions ultimately cause more harm than good.
The true hero of the play, if there is one, is arguably Mark Antony. Antony is able to manipulate the crowd to turn against Brutus and Cassius and ultimately to win the civil war. He also proves himself to be a shrewd and capable leader, and he ultimately restores peace and order to Rome.