To be honest the spookiest part is the ending.
For people not aware, you can wind up the animatronic's suits so that the metal bits are put away, as such you can put them on like a fursuit. Thing is you gotta keep 'em wound up. Well, at the end of the game, a character gets into the suit that's been haunting you the entire game- 'CLEVERLY' named Springtrap- but he forgets to wind it, causing the inner parts of the suit to spring and gore him.
The game itself isn't that scary, but going out like that? Euugh. It's worse because you see it's not like a sword to the brain, it dosn't instantly kill him. You see him fall to his knees bleeding out like that. Atari style graphics, but still.