The psychology of it is that movies like "Passion of Christ", "Schindler's List", Saw I through XXVI, etc is that it gives the mind the ability to experience something horrific that would be traumatic in real life (and thus the fear in seeing it or experiencing it) from a safe place. The movie can't hurt them so they experience that which would, in real life, horrify and terrify them from a safe and secure point of view.
This viewing can allow viewers to "survive" a film with the same release of dopamine and pleasure chemicals the person would experience if they survived the experience in real life circumstances. They are surving the ordeal vicariously but it still has some of the same effects.
It goes even deeper than that to a primal level of disgust that originates in a part of the brain that has kept humans from eating and coming in contact with rotten meat, feces, and other objectionable things that allows them to see these disgusting things and acts without the smell, feel, etc. It's a showcase of things to avoid inside your brain.
And lastly, the curiosity we experience when we drive by a car accident and everyone has to take a look, called "rubbernecking". It's so common and instinctual, we actually have a word for it. Movies like this are a 2hr rubbernecking experience without the stiff neck and distraction of the road.
People that have truly seen horrible things or dealt with people who have committed these gruesome crimes often have little desire to see these movies while interestingly, people that have not tend to enjoy these movies.
I remember being just completely disappointed and laughably disgusted by the movie Fight Club. I was in a NYC city bar listening to guys talking tough and how much they loved the movie. I tried explaining to them that the movie was lame. But because I had broken noses, I broke noses, split heads open, and it was always miserable with much guilt afterwards. It wasn't "cool" at all. I had met people with violent multiple personality disorder. It is not cool. I could tell right away that these guys had never been in a fight in their lives or met a pathologically insane person. So they thought the movie was cool. While I lived it, they saw it from the safety of a movie theater seat.
That's the hook. And once desensitized, people need greater and greater levels of gore and macabre to illicit the same response. Ever notice that sequels within the same horror franchise get just more gore and bloody from one sequel to the next? Now you know why.