King can be a genius sometimes. In the book, someone tries but narrowly fails to save someone's life and instead sees them die. The would-be rescuer relives the terrible incident in dreams and flashbacks. This is classic PTSD, which King describes perfectly without naming.
(A character in Salem's Lot also has signs of PTSD after losing his wife in a motorcycle accident. He keeps picturing her boot lying in the road afterwards.)
Years ago I worked with a woman who, when we got chatting, spontaneously told me she'd once seen someone die in the same circumstances. (This was after I'd read the book but we weren't discussing it.)
The victim was a toddler, of a similar age to victim in the book. The woman had only seen what was going to happen at the last minute, and couldn't save him. She was still getting the dreams and flashbacks after I think about 2/3 years.
I told her, 'That's PTSD. You MUST see your doctor. You're not going mad, it's a natural reaction, but you can't go on like this. Get some help.'
One of King's major themes is the longterm mental effects of physical and emotional harm done to children. Writers can be prophets.