An "antagonist" is someone in opposition or conflict. In literary use, the antagonist ("bad guy") is the person who is opposed to the protagonist ("good guy"). An example of this would be the Joker as an antagonist of Batman.
The antagonist are Blackie, the gang itself and the socity. Blackie is antagonist because he was a lider of the gang but Trevor replace him. 'Old Misery' portrait as an antagonist when he want to return to the house.