What is the main rhetorical device used in "The Ones Who Walk Away from Omelas" by Ursula K. Le Guin?

The main rhetorical or persuasive device Le Guin uses in "The Ones who Walk Away from Omelas" is pathos, or an appeal to emotion.
Le Guin would like us as readers to have an emotional response of dislike and revulsion to the idea of Omelas, a city based on the ideal of "the greatest good for the greatest number."
Imagery is the best rhetorical strategy to use for eliciting an emotional response. Imagery is description using the five senses of sight, sound, taste, touch, and smell. We tend to react to details we can specifically see, hear, smell, and feel.
Le Guin first describes the beauty of Omelas, drawing us in to perceive this land as a lovely place, such as in the following quote:

With a clamor of bells that set the swallows soaring, the Festival of Summer came to the city Omelas, bright-towered by the sea. The rigging of the boats in harbor sparkled with flags. In the streets between houses with red roofs and painted walls, between old moss-grown gardens and under avenues of trees, past great parks and public buildings, processions moved.

She then shows us what all the happiness is based on: the suffering of an innocent child. But she doesn't simply tell us that all of this is based on the suffering of an innocent child. She shows us the child and provides vivid details of its life:

The child used to scream for help at night, and cry a good deal, but now it only makes a kind of whining, "eh-haa, eh-haa," and it speaks less and less often. It is so thin there are no calves to its legs; its belly protrudes; it lives on a half-bowl of corn meal and grease a day. It is naked. Its buttocks and thighs are a mass of festered sores, as it sits in its own excrement continually.

The details repulse us and cause us to have an emotional reaction. Suddenly, all of Omelas seems stained and tainted by the existence of the cruel truth of an abused child at its core. Le Guin wants us to feel that it is not all right to base everyone else's happiness on the suffering of one person, and descriptive details are the main way she makes her case.