The novel is set against the backdrop of slavery, and the atrocities committed during that time are depicted in a way that evokes horror. The psychological state of the characters, who are constantly haunted by their past, also contributes to the horror aspect. For example, Sethe's struggle with her memories and the appearance of Beloved as a manifestation of those painful memories makes it a horror novel.
The ghostly presence of Beloved. She is not a normal character but rather a haunting figure that represents the past traumas of slavery. Her very existence in the story creates a feeling of horror among the other characters.
There are multiple factors. The way Morrison writes about the violence and dehumanization of slavery is terrifying. The character of Beloved, with her strange behavior and the mystery surrounding her, adds to the horror. Also, the overall atmosphere in the novel is one of unease and dread. The characters are constantly in a state of emotional turmoil because of the past, and this is presented in a way that gives the reader a sense of horror.