3 ideas
The Spanish twins Ric and Rac throw the Huttens' white bulldog overboard. Echegaray, a poor Basque woodcarver travelling in steerage among hundreds of deported sugar workers, leaps in and saves the dog, but he drowns. First-class passengers treat his death as a minor disturbance, while the dog's owners fuss over their pet, which exposes how the ship ranks the value of a life.
The book argues that cruelty and fanaticism succeed less through villains than through ordinary people who tolerate, excuse or quietly benefit from them. No single passenger commits the great crime. Instead, each small accommodation, such as laughing along, looking away or changing a seating chart, becomes the permission that larger evil needs.
Put people in a closed space for a fixed period with assigned tables and cabins, and remove any exit. Under those conditions the everyday tactics that hide contempt, such as avoidance, changing company and leaving the room, stop working. Hierarchies of nation, race, class and sex then have to be acted out in plain view at every meal, so the arrangement works as a diagnostic tool on society in miniature.