One of the most messed up Stephen King novels is 'Pet Sematary'. The idea of a burial ground that brings things back to life in a distorted way is truly disturbing. The family's grief and the consequences of using the cemetery are just nightmare - inducing.
'It' also falls into this category. The shape - shifting entity that preys on children, exploiting their fears. The whole concept of a town haunted by this malevolent force that resurfaces every few decades to continue its reign of terror is extremely messed up. It delves deep into the childhood traumas and fears that can follow people throughout their lives.