What might a utilitarian say to try to justify the Omelas system?
A utilitarian would point to how much happiness the citizens of Omelas experience in order to justify its system of governance.
In assessing an action's value, the utilitarian uses the justification of how much good is expanded to as many people as possible. When philosopher Jeremy Bentham explains the concept of "utility," it is rooted the expansion of "happiness:"
Nature has placed mankind under the governance of two sovereign masters, pain and pleasure... By the principle of utility is meant that principle which approves or disapproves of every action whatsoever according to the tendency it appears to have to augment or diminish the happiness of the party whose interest is in question: or, what is the same thing in other words to promote or to oppose that happiness.
Bentham suggests that the "measure of government" is to "promote the happiness of society."
This happiness principle can be used to justify the structure of Omelas. The system of Omelas is one whereby all "happiness" is dependent on the child's "abominable misery." The suffering experienced is far less than the happiness that is gained from it. Society is able to experience pleasures such as knowledge, beauty, harvest, and skills because of the child's predicament. The result is that more people prosper and experience happiness in Omelas. This utilitarian would use this metric to justify the Omelas system.
Utilitarianism is defined as a belief that an action is right if it is useful ("utilitarian") or if it benefits a majority. Further, it can also be defined as a belief that an action that promotes happiness is right, and that we ought to behave in a way that promotes the greatest happiness for the highest number of people.
In Omelas, everyone is happy except for the malnourished and miserable child who is left to suffer horribly in the broom closet; the only possible exception to this would be the individuals who, for whatever reason, decide to leave Omelas, but it seems clear that this number is a very small minority. The suffering of one person, then, promotes the happiness of the vast majority of the remainder of society, and so a utilitarian would defend this system. A utilitarian would likely say that it is sad that one person must suffer, certainly, but because this is the condition on which the happiness of thousands of others rests, this single person's suffering is justified.
Utilitarianism is based on the idea of the greatest good for the greatest number. A utilitarian would therefore say that making one child suffer horribly to insure the happiness of the rest of the society is a reasonable trade off: many are made happy because one is not. The greatest good has been secured for the greatest number. One child's misery is not so high a price to pay.
The story, of course, invites us as readers to question the morality of utilitarianism. We are shown the squalor and loveless misery the child lives in and feel appalled by it. Some people in Omelas can't accept this trade off, despite the benefits their society offers them, so they walk away, seeking something better than a hard-hearted logic in which their happiness is based on another's suffering.