Brutus disregards the advice of Cassius and allows Antony to speak. Why?

Brutus knows their situation is questionable, the people will be angry about Caesar's death, and Antony's presence may help to calm the chaos. Brutus believes that if he speaks first, and Antony agrees to only make positive comments, all will be well. Brutus makes the mistake of believing Antony honorable, of believing that he will keep his word.... he misjudges him.