Antony keeps referring to Brutus as an "honest" man because he's being sarcastic. What he says and what he means are two completely different things. He's prodding at Brutus' guilt..... trying to get under his skin. The fact that Brutus really is honorable makes the sarcasm even more potent. None-the-less, the first time he attributes honor to Brutus, the audience is with him..... but by the time Antony's done, the audience is questioning that honor.. not applauding it.