> "Human dignity does not depend on a person's abilities"

That should be a hard line no one is allowed to cross. If you do, no logic will keep you from ending up in a very bad place.

For instance, I'm a big fan of Terry Pratchett's Discworld stories, where his humorous and compassionate tales comment on much of human experience. It's a fantasy world with wizards, trolls, dwarfs, vampires, etc.

In one book he goes into ghouls - obscure, despised and persecuted almost to extinction, but in the story it is found they secretly make beautiful ethereal music. So a concert is arranged to present the music, and all is well, the end. Except for my doubts - I mean, if they didn't make nice music, then it would be alright to dispose of them?

Benthamite utility has a particularly ugly underside to it, and it is easily converted into "disposal of negative value" of the nastiest kind. Which can only be stopped by a moral hard line, as stated.