Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Changes
Try Ebenezar McCoy casually ripping the life from 200 men with two waves of the Blackstaff. This is why we have the Laws of Magic, folks. You know it's bad when even Harry is appalled at the sheer cold-bloodedness of it.
The Red Court attack on the FBI building. Made especially disturbing by Susan's explanation of their methodology: start from the ground floor and kill their way up, taking out anyone they find, solely to "send a message."
It's also pretty chilling that she states that the human Cartels use this tactic too.
The heart-ripper curse was powerful enough to take down every Red Court vampire on the planet. All of that power was garnered by Human Sacrifice. Harry may have saved the particular lives he'd been fighting for, but a hell of a lot of innocent people were butchered before he even got to Chichen Itza.
This was mentioned on the Fridge Horror page, but it deserves to be put down here for how terrifying it is - remember during Summer Knight when the vampires said they would cease-fire if they were given Harry? Odds are, they would have held him captive (possibly for years) while they prepared the spell and sacrificed him at Chichen Itza to get rid of Ebenezar! *shudder*
Nightmare Fuel even secondhand: Susan and Martin, finding the dismembered bodies of Maggie's foster family, three of them young children. And then having to re-assemble them like a jigsaw puzzle just to determine if Maggie is among them, their Horror Hunger urging them to gorge on the blood that's been spewed everywhere and is seeping from the mangled body parts. Good Lord!