White Society in General
Mr. Max (and author Richard Wright) argue that although Bigger is a murderer, it is white society and the system of oppression created by that society which turned Bigger into a criminal. In that case, the antagonist would be the system that oppresses Bigger and keeps him from having options in life aside from poverty and crime.
On the one hand, Mr. Buckley is simply doing his job: prosecuting criminals for the state. On the other hand, it’s clear that he deliberately uses Bigger’s case to gain reelection by proving to white citizens of Chicago that he’s on their side and he’s tough on black criminals.