This is an interesting question because it accurately points out that it is not normal for a story's protagonist and antagonist to be the same person. By definition, a story's protagonist is the central figure of the story. Notice how it doesn't say anything about the protagonist being the "good guy" or the person that the audience considers the hero. Monster is a story about Steve Harmon, his involvement with a crime, and the following trial; therefore, he is the story's protagonist. An antagonist is a character that stands in opposition to the protagonist. More often than not, a story's antagonist is considered the "bad guy;" however, Steve is his own worst enemy. It's possible to consider the prosecuting attorney the antagonist, but the story's main conflict is an internal conflict that Steve has with himself. He struggles throughout the narrative to justify his actions and see himself as not a monster. In terms of his overall mental and emotional health, Steve is his own worst enemy.