What makes a horror novel disturbing and insane?

It depends. Some might find them disturbing because they often deal with mental illness, which can include dark and difficult themes like hallucinations, severe depression, and abnormal behavior.

The unknown. In insane horror stories, there are often elements that are not fully explained. Like in a story where a person keeps seeing a strange figure in the corner of their eye but when they look directly, it's gone. It creates a sense of unease and fear because our minds try to fill in the blanks.

Taboo horror stories are disturbing because they often touch on things that society considers unacceptable or immoral. For example, stories about incestuous relationships within a horror context are deeply unsettling as they go against basic moral norms.

The unpredictability. Schizophrenic characters often act in ways that are difficult to anticipate because their perception of reality is distorted. This can lead to sudden and shocking actions or reactions in the story.

Well, body horror novels are disturbing mainly because they violate our sense of self and physical integrity. Our body is something we identify with, and when we read about it being violated, invaded, or transformed in unnatural ways, it creates a sense of unease. Take a novel where a character's body is slowly being consumed from the inside by some unknown force. The thought of that happening to our own bodies or even just seeing it happen to a fictional character makes us squirm. Also, body horror often uses the body as a battleground for some greater horror, whether it's a supernatural power or a mad scientist's experiment, and this combination of the body and extreme horror concepts is what makes these novels so disturbing.

One major factor is the violation of the natural order. In sick horror stories, we might see things like the dead coming back to life in a wrong or corrupted way, or the innocent being targeted in the most brutal fashion. The sense of helplessness it instills in the reader or viewer is also a big part. When we see characters being subjected to these sick and cruel situations and they can't escape easily, it makes us feel uneasy and disturbed.

The combination of innocent babies and horror elements creates a creepy contrast that gets under your skin.

The combination of exposed flesh and horror elements can create a very uncomfortable and creepy feeling for viewers/readers.

The isolation. Insane asylums are often in remote locations. In a horror novel, this means the characters are cut off from help. For example, in many such novels, when the protagonist realizes something is wrong, there's no easy way out.

One such novel could be 'House of Leaves' by Mark Z. Danielewski. It has a very disturbing and complex narrative structure that can seem almost insane at times. The use of footnotes and different typefaces adds to the sense of unease. Another is 'The Haunting of Hill House' by Shirley Jackson. Although not strictly in diary form, it has that sense of a creeping insanity as the characters experience the horror of the house. It's a classic in the horror genre.