This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Sleepaway Camp
Just admit it. The ending scared you shitless. The plot twist, Angela decapitating someone, and the fact that she stood there, frozen and breathing heavily.
The scene where the aunt tells calmly that "Angela will be a lovely name for him" to Peter. Think about it: she's forcing her traumatized nephew into pretending he's his sister just because she already has a son. Bitch needed to DIE.
Judy's horrific death scene. She's held down with a pillow while Angela grasps a hot curling iron before shoving it into (most likely) her vagina. Only the shadows are shown, but you can hear her skin sizzling as the iron is pressed inside.
Though he may have deserved it, Artie's burns (pictured), leaving him blind and unable to do anything but scream in wretched pain, can qualify.