The killing of Caesar was a moment wherein he realized that the plot to assassinate him was so intense it included most of his closest friends. Caesar was definitely heartbroken on his death because even Brutus was included as one of the conspirators.
After his death, Antony grieved his leader's bloodshed but kept a gentle attitude in order for the conspirators to trust him. This Act defines that Antony will be one of the people who will plot the revenge for Caesar; as Brutus underestimated his potential and well-being.