An antagonist can take on many different forms. When adhering to the idea of external conflict (which pits the protagonist against the antagonist), an antagonist can be man/woman, supernatural, or of nature. Included in the external conflict of man verses man, society or groups can be deemed the antagonist.
In regards to Walter Dean Myers novel Monster, the antagonist in the novel can be defined as a few different entities.
First, King is an antagonist. Steve Harmon (the protagonist) is pitted against King. Both King and Steve are on trial for murder. It is only through separating Steve from King that Steve's lawyer believes Steve has a chance of being found not guilty.
Second, Steve's antagonist is the justice system. Steve's lawyer tells him that her job is to make him human in the eyes of the jury. Going into the trial, the jury only knows three things: Steve is young, black and on trial. Essentially, this makes him guilty.
Not only is the jury seen as being prejudiced, the court officials do not give Steve the benefit of the doubt. Instead, they bet on his guilt and only hope the trial continues in order to make money.
Steve also faces an internal antagonist: himself. Steve no longer recognizes himself when looking in the mirror. Instead, he sees a monster. In order to make it through the trial, Steve must not give up and believe himself to be the monster everyone is calling him.
In the end, many antagonists exist within the pages of the novel. While some may argue that Steve's inner self is the most important antagonist (given the importance of him not becoming a monster in the eyes of those around him), others may state that the true antagonist is the justice system. Contradicting this is the fact that Steve is actually found not guilty. Therefore, the antagonist for the novel is based upon the individual reader's interpretation of who the true antagonist is.