“Mischief, thou art afoot; Take thou what course thou wilt.”
A friend, benefactor, and father figure has been murdered. Caesar was not killed like a “sacrifice” as Brutus desired, but literally hacked to pieces.
Caesar picked Mark Antony up when he was down, a runaway, deeply in debt, and then tutored him to become his second-in-command. Caesar entrusted Mark Antony with his armies as well as his political fortunes while absent from Rome. He welcomed Antony into his own household as one of the family.
Seeing the body, Antony is filled with feelings of loyalty and gratitude to Caesar, and anger and vengeance toward Brutus and Cassius. Mark Antony nevertheless gives Brutus and Cassius a chance to give reasons “why and where Caesar was dangerous.” None are forthcoming.
Emotions run high, but Mark Antony, the battle-tried general, takes over. In the course of a very short exchange over Caesar’s body, he quickly forms a plan to move the people of Rome to revolt, while maintaining a calm demeanor with Brutus and Cassius. His training in Greek rhetoric comes in handy.
Surprisingly overly trusting of Antony (much to Cassius’ dismay), Brutus gives him the perfect opportunity. Having summoned a crowd, Brutus turns them over to Mark Antony, and exits. The rest, as they say, is history.