This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Falling Down
The story Foster tells to the family after he crashes their barbecue, that ends with him and his family "all sleeping together in the dark", which seems to imply that Foster intends to kill his wife, his daughter, and himself once he gets to them. The dreamy way he tells the story just makes it worse, as he genuinely seems to feel that is the right thing to do.
The whole movie becomes this once you get to the end, in part due to Paranoia Fuel. Given that many people find Foster sympathetic, it makes one wonder just how easily this could actually happen to them or someone they know.
The Whammy Burger scene can be this for some, considering that this could happen to anyone.
The Neo-Nazi freak whose favorite possessions are an anti-tank weapon and a used can of Zyklon-B.
The Latino gangbangers try to kill Foster in a drive-by shooting. They miss Foster, but do hit several other people by accident. Imagine being one of the innocent bystanders who got shot, out of the blue, without knowing why.