Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Never Where
This exchange cements the fact that Croup and Vandemar decidedly aren't human.
Croup: If you cut us, do we not bleed? Vandemar: ...No.
The duo's habit of eating everything alive. Rats, cats, humans, basically anything with a pulse. At one point, Vandemar finds a sandwich and instead of eating it, uses it as bait to catch pigeons and eat them whole.
The whole of Richard's Ordeal, but especially the train carriage full of rotting corpses, every single one a suicide.
The idea of being permanently invisible to everyone around you. You can't withdraw money, your fiancee of 18 months doesn't recognise you, your desk at work's been removed, you can't hail taxis, your house gets sold while you're still living in it...
Whereever Anaesthesia went on Knightsbridge.
Mr Croup gleefully eating an object he had doted on with apparent love and affection mere seconds before is so horrifying it borders on Nausea Fuel.
The Big Bad the Angel Islington is extremely unnerving as a being so utterly self-obsessed and self-righteous it's gone well past the point of insanity.