In "The Ones Who Walk Away from Omelas" it is possible to think of people as being divided into three groups. The child who is locked away, the people who see the child and are complicit in its torture, and the ones who walk away.
The story centers around the idea of a minority (any minority, not just racial or cultural) in a society serving as a scapegoat. A scapegoat is someone or something who takes the blame for the wrongdoing of someone else. In other words, although the wrongdoing is not actually the scapegoat's fault, he or she is punished for the wrongdoing of another person or group. In the case of Omelas, the scapegoat is the child who is locked up, and the rest of the people in the story know that in order to maintain their own health and well-being, they must keep the child in this deplorable condition.
The child represents the neglected minority in a society. The child is symbolic of any abused, mistreated, or ignored minority in society. For example, you could look at the Jews in the Holocaust as a possible interpretation of this child. They were imprisoned and killed while millions of other people were allowed to roam free because of a simple religious distinction.
Most of the people in the story see the child and understand its misery, and they understand that it must stay miserable in order to allow them to be happy. This symbolically represents people who know that a minority is being mistreated but choose to ignore this mistreatment because it does not affect them directly. To continue with the example from above, this could represent the millions of citizens during World War II who went along with Hitler's tyrannical rule and did nothing to stop the Nazis.
The third category is the people who "walk away" from Omelas. They see the misery that one person has to suffer, and they decide that even one person suffering is not worth their happiness. In the story, these people decide to walk away. In real life, in the example we are using, this could be any of the people who stood up to the Nazis and tried to resist in some way.
The Nazi example is just one possible interpretation, but the same reasoning could be applied to any such situation. There is always injustice happening in the world to minorities. Some people choose to ignore it, and others choose to actively fight it.