The problem is, I can't think of anything worse. There's something particularly terrible about mass industrial slavery. Maybe some passing atrocitologist can help.

I'm not quite an atrocitologist so I have no idea whether some of these things I can think of were actually ever put into practice, but I can think of lots of things worse. I can also guarantee you with 90% confidence that there's a lot of manga (especially doujinshi) out there that do picture things you'd consider much worse, especially when you delve into the darker circles. Some japanese artists have literally become world-renowned 'experts' on the topic of fictional mass atrocity.

I'm not comfortable discussing specific examples without a wall of spoiler prevention features requiring the viewer to pass a mental fortitude test to view the content. I might have mentioned this before, but I've once had an acquaintance bend down and vomit on the spot upon recounting one of my more horrible nightmares. I try to avoid dishing out such mental damage on unprepared individuals nowadays.