The story develops some profound, timeless questions about human nature, human society, and the individual's relationship to society.
The child who is imprisoned and abused beneath the streets of the city (and who dies there, to be replaced by another innocent) is a scapegoat. He has been singled out through no fault of his own and isolated from society to suffer so that their collective well being will be assurred. As in real societies that have made scapegoats of certain citizens, the people of Omelas choose the most powerless among them (in the story, disabled children) to be their scapegoats, one after another.
The individual's relationship to society as explored in the story is shown in those who walk away from Omelas--and those who don't. After witnessing the child's misery and after understanding that personal happiness in Omelas comes only from his suffering, some citizens (young and old) choose to leave the town. It is a moral choice. Those who walk away from Omelas reject evil, and they refuse to condone it by remaining in a society that promotes it. They reject happiness purchased by another's suffering. Some make this moral choice immediately upon first seeing the child. Some struggle with it for years, remembering the horror they once had seen, before they can endure it no longer and must walk away. Those who leave Omelas don't know where they are going or what their subsequent lives will be; they only know their own humanity tells them they cannot live as they are expected to live in their society.
The individuals in the story relate to their society in one of two ways. Some choose to live with the evil they see--embracing it, rationalizing it perhaps, or trying to ignore it. Others are deeply affected and walk away from it. In their reactions, we examine our own relationship to society. When presented with "a child," would we make the moral choice? Human history is filled with examples of those who made such difficult moral choices, and these are the people we admire for their courage and integrity.