Robin's death in the opening sequence. The already somewhat creepy abandoned building the tragedy takes place in is bad enough, but the actual event is utterly haunting. No one should have to go out like that, especially not a child.
There's also the fact that such a violent death was simply the result of a bunch of overzealous children playing a game.
Even then, the kids chanting "The killer is coming! The killer is coming!" is more than a little disturbing.
The killer making phone calls to each of the targets. The creepy voice he uses, combined with the Dutch angles employed to hide his identity make for an unnerving introduction to the film's villain.
Lou's death. While it's hard to say that he didn't deserve what he got in the end, he still gets arguably the most violent death in the movie, and it happens in full view of a crowd. The Epileptic Flashing Lights don't help.
This one was real-life Nightmare Fuel for David Mucci, Lou's actor; he suffered from an unknown medical condition that required him to be immobilized for the scene where his head lies on the catwalk, and director Paul Lynch held his hand throughout the scene to reassure him that everything would be alright.
Wendy's chase scene through the dark halls and rooms of the school is pretty terrifying, and lasts for several minutes. The fact it is happening in the same vicinity as her classmates who are dancing and having fun, with the music playing distantly in the background, adds an extra layer of disturbing to it.
