This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Vanishing
The ending. When Se7en was first released, one critic said it featured the most frightening ending of any film since this one. They were bang on the money.
The core premise, in a very Adult Fear-sort of way. Your girlfriend goes to the shops - and simply never returns.
When Fridge Horror sets in.... oh boy. When he's first trying to put his plan in action, Raymond is stymied again and again, either by his own amateurish mistakes, or by simple bad luck. Just before he meets Saskia, he is ready to give it up, but he doesn't appear to regret it. He is laughing to himself, suggesting he's thinking, oh well, he gave it a shot, and he had fun. Fast forward three years, and Raymond is about to do the exact same thing to Rex. Rex looks like he won't go for it at first. Raymond loses his patience, starts to become angry, and presses Rex into going through with their deal, that Rex will find out what happened to Saskia, but only if he's willing to share her fate. The change in attitude is telling. Raymond now has a taste his new little hobby. He's a budding serial killer. He may be caught eventually, but it will be many years — and many bodied buried — before that happens.
The look on Raymond's eyes when he's looking at a desperate Rex who can't decide whether he is going to drink the first's soporific. This is the first time we see Raymond as the sociopath he really is. And giving that he is a perfectly realistic exemple, it is all the more terrifying.