The whole story is built base on a simple concept:
“What if the prosperity and happiness achieved by the suffering of one child.”
One of the metaethics being used is utilitarianism, in which the right action is determined by the consequence. The right action right action is such that it results in the most benefit for the most agents.
Despite knowing about the existence and suffering of the child, no one would do anything about it. They afraid that their action might cause huge disrupt. So they shifted their mindset, making up excuse to not act for what they think was wrong.
Mean while, a person with Kantian Ethics would act accordingly to the universal law. Does not matter how much benefits it bring, the action of making a child suffer is morally wrong.
The story actually mirroring our reality, what is different is that, instead of being able to see and making contact with the child like in Omelas. The consumerism alienates us from the process of producing the goods we use.
How about the ones who walk away from Omelas? They might have recognized the evil of the society they living in, and decided to act differently. It is not morally right for them to stay or to save the child. Or it is a lesson for us to avoid the psychological trap of “Someone has to suffer in order for us to have a great society”.