The Ones Who Walk Away from Omelas by Ursula K. Le Guin (1973)

Picture a city in summer, bells ringing, swallows over the rooftops, horses garlanded for the festival race, a place so happy you suspect at once that it must be a lie. It isn't. Le Guin then shows you the one cost: in a broom closet beneath the city sits a child, naked, fed on a half-bowl of cornmeal, never spoken to, and everyone in Omelas knows it is there. The whole radiant happiness of the place rests on that single, unredeemed misery, and they have agreed to it. This is the most quietly devastating thought experiment in American fiction, a story that hands you the oldest bargain in ethics and watches your face. Most stay, having reasoned themselves to peace. A few rise from the table and walk out the gate, on into a darkness even the author cannot describe. You will know, before the last line, exactly which one you are.