A person with mental illness may have no idea why they committed their crime or they may report reasons that are illogical or incomprehensible to the rest of us. Things that don't make sense raise our fears because we feel less able to predict the action of others and less able to protect ourselves. If a mentally ill man can stab a total stranger after a ball game, shoot at the president to make a point with a movie star he's never met, or drive half way across the country to shoot up Congress because of cats or a failed mine, then anything is possible and our fear increases. We become very interested in the story, in part, hoping to find logic, increase predictability, and improve safety. Healthy brains gather and organize input from our senses, translate those stimuli into awareness, filter out unnecessary input, connect old and new information, develop understanding, organize everything, store all of this so that we can retrieve it later, and much more.
Mental illness is disease of the brain wherein brain processes are profoundly altered.
Like personality, there is considerable variation in brain disease. A diseased brain can totally misunderstand the words and actions of others - the movie star is thinking about me. A diseased brain can hear and feel things that most of us would say don't exist - I'm getting messages from the satellite dish. A diseased brain can hatch complicated schemes that don't make any sense and are based on totally illogical premises. In many respects, people with mental illness are just like you and me.