The dudes who killed Caesar didn't want people stirring up trouble and making people want to avenge him. So if he was going to be allowed to make his little speech, Antony had to pretend he wasn't going to do anything. So he said he wasn't going to praise Caesar. Instead, he "praised" the assassins, only he did it in a super sarcastic way and made people think they were jerks, which is exactly what the assassins didn't want. Basically he just wanted them to let him make his speech so he could get himself some support.