This scene is just after the scene where Anthony changes the thoughts of the Roman public that Caesar was killed unlawfully. The Roman pubic a furious with the conspirators. They find some one in the street and ask him questions and the answers he gives them leads him to be killed. He is Cinna the Poet and they confuse him with Cinna the Conspirator. I can tell they are really angry with him as they say ” Tear him to pieces, he’s a conspirator.”