Steve Harmon is accused of felony murder alongside James King, and he tells the story of his difficult experience in jail while he is on trial for his life throughout the novel. Steve Harmon is depicted as a conflicted adolescent who struggles to determine his own guilt or innocence during his time spent in jail and on trial. During the trial, Osvaldo Cruz and Richard "Bobo" Evans testify that Steve Harmon was involved in the robbery and murder of Alguinaldo Nesbitt, a Harlem drugstore owner. Richard "Bobo" Evans testifies that Steve's job was to enter the drugstore and make sure the coast was clear before the robbery. There are surveillance photos in the novel that confirm that Steve entered the drugstore before the robbery, but his participation in the crime remains ambiguous. For example, Steve was supposed to give a signal that the coast was clear (which he did not give), and the fact that there was also an older woman in the store during the robbery implies that he did not participate.

Following the robbery and murder of Mr. Nesbitt, Steve did not contact Richard "Bobo" Evans or James King and never received a share of the profits from the robbery. He immediately distances himself from Osvaldo Cruz, James King, and Richard Evans following the crime. During the trial, Steve Harmon presents himself as a morally upright person and good student who is not associated with the other criminals involved with, or accused of, the crime. At the end of the novel, Steve Harmon is acquitted, but the stigma of a "monster" remains attached to his character.

Monster by Walter Dean Myers is s study of guilt by association.  It is also a poignant telling of how a person can lose sight of themselves when they allow society to label them.

Despite the fact that Steve is acquitted of any involvement in the crime that was committed by his two acquaintances, he is still stigmatized by having been associated with them.

After the robbery, Steve finds himself accused and jailed despite the fact that he did not participate in the actual crime. He spends his time writing in his diary and trying to keep track of his innocence. This is difficult to do, given his proximity to the crime scene and the potential that he did - willingly or not - appear to play the role of lookout. 

Upon his acquittal, he approaches his lawyer in order to demonstrate his gratitude. However, instead of embracing him she redirects her attention elsewhere. This leaves Steve with the uneasy feeling that he will always be labeled a "monster."