Horror, more context would help, but...
Cannibalism and such...
- After spending some time at an inn and enjoying the meals, going out to the cold storage and finding that humanoids are being butchered and turned into meat pies, etc.
- A convalescent home or religious order that tends to the injured. And turns out slows the healing of the patients just for the anguish and pain it causes.
- The old granny of the neighborhood who makes treats for the homeless; from the homeless (i.e. as above), from the dead bodies in the graveyard...
- The use of the dead from a city being used as fertilizer
Children...
- kidnapped and being experimented on to separate them from their innocence or such (i.e. think of the Golden Compass storyline).
- stolen from parents and sold for experiments, slavery, turned into assassins, used as arcane batteries (i.e. the Matrix)
- Children/Child as the BBEG
Sewage...
Helplessness...
- An injustice/evil/danger/etc that the party is helpless to prevent
- Forcing the party to chose between the lesser of two evils, such as feeding the infirm to a vampire or causing a war where thousands will die