“I am different not less.” – Temple Grandin

People with mental illness and people with a disability are no different from anyone else. They do the same thing everyone else does. It is only that life and chemistry makes their actions extreme. A person may hear voices. You may not, but you do have an internal monologue that talks to you. There are many people in a mental hospital that cut themselves. Many people do not. But they do self-destructive things. Everyone thinks of suicide. Because of depression. Chemical imbalances make that desire stronger.
Children with autism may twirl their fingers. It is self-stimulation. A way to calm down. If everyone took note, they would see something they do similar things. Hair twirling. Pen cap chewing. Beard stroking. A child with autism may become violent. People have violent tendencies. Because they know of social conventions, they suppress it. A child with autism may hit a teacher. His or her reactions are extreme, but everyone has physically fought with a sibling (maybe at age 2). But they are in school, they know that is not social correct to hit a teacher. That they will get in trouble.
It is difficult, but everyone needs to learn that mental illness is just that. An illness. We don’t make fun of a person with cancer. We do not shun them or think them as weird. Mental illness is not the person’s fault.