Nightmare Fuel / I Live in Your Basement

• A "girl" turns inside out, described in full detail.
• The main protagonist Marco caves Keith's skull in with a paper weight. Keith falls to the ground and lays there unconscious without breathing for a moment, which Marco is horrified by.
• Marco's head injury after he is smacked by a softball bat and the full detail of how he feels counts as well. Right after he is hit, the details of his injury are just unsettling to read through: the ground tilted, the pain exploded in Marco's head, everything turned bright red (which actually forced him to shut his eyes), and the ground swallowed him out of his consciousness. It's not only scary, it's incredibly PAINFUL! Just imagine that pain! The fact that it's told in first-person gives the reader a vicarious experience of this kind of pain, which makes it even worse! Made even worse by the fact that Marco is only 12 years old, and as you would know, very few people around that age or younger have survived getting struck on the head by a bat, especially after the full-blown detail of the injury mentioned above.
• The boy, Keith, also elicits very strange and disturbing stalker vibes, especially considering how young the protagonist is. Then there's the fact that the story descends into a complete Mind Screw after Marco gets hit in the head with a softball bat. While it can be interpreted as Marco suffering from his concussion, the end actually reveals that Keith was having a nightmare about being a human and living above the basement.
• The fact monsters live in basements and do not dare emerge elicits some likewise very disturbing vibes.
• The whole story is just a long series of contradictions; even the aforementioned Twist Ending makes very little sense, in context or otherwise.
• At one point after a bunch of bizarre happenings Marco's mom takes him to the doctor. After Marco explains what's going on, the doctor just calmly says that they need to remove Marco's brain and examine it under a microscope. Marco is absolutely terrified, but neither the doctor nor his mother act like there's anything weird about this. It turns out this part of the book is All Just a Dream, but it's still pretty weird and scary.