This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Happy Appy
Two posts into the first story, we have the episode of the titular show that depicts the events of 9/11 two years before the attacks take place. It begins rather innocently with Happy teaching kids about the circle of life in frogs and plants. After a scene of Happy and the kids in the playground, the screen starts filling with smoke. They turn around to see two towers burning up in front of them. Then, it shows a child crushed by a piece of the plane. Happy and the other children just stand still and watch the crushed boy as he's screaming in pain, while other children try to lift the piece off. After this, the children that were with Happy ask him why people are falling out of the towers. Happy turns to the camera with a death smile and coldly says "That's natural, children", drags the children away, and leaves the crushed boy to die.