Crazy people seem to be quite popular in mystery/thriller,you know- the insane ones. Crazy men wielding axes and stuff like that. Kidnappers with a murderer's smile. Some books that include these type of people try and tackle the POV of the psycho. Some succeed... Some fail.
You see when writing inside the brain of someone who is obviously mentally disturbed, you have to normalise their actions. See it from their point of view. They just killed someone, they can't be all shocked and stuff. Doing that is normal to them. Every horrible thing they do is like doing a household chore- normal.
To get a better understanding, do some research on the mindset of a killer/ kidnapper. I'm sure there's some stuff out there to help you write a convincing account. These type of POVs can make a good M/T book amazing if done right. It will leave the reader with chills, leave them amazed.