Behavior has reasons not causes. Persons are not reducible to brain chemistry.
This is what the late Thomas Szasz spent years courageously trying to explain to us. He was routinely misunderstood, misstated, and maligned. Contrary to common misconception, Szasz did not deny that people do "crazy" things, only that action -- purposive conduct -- is illness. He did not rule out that some people who do "crazy" things may have brain illnesses, even ones yet to be discovered. But he rejected "mental illness" as a category mistake because the mind is not an organ. ("Mind is a verb not a noun," he he once told me.) He liked this passage from the philosopher Gilbert Ryle in The Concept of Mind:
A myth is, of course, not a fairy story. It is the presentation of facts belonging to one category in the idioms belonging to another. To explode a myth is accordingly not to deny the facts but to re-allocate them.Szasz spent his life restoring the facts of human action to the category in which they belong. For an unlimited number of reasons, people are capable of drawing conclusions about the world and themselves that in their view justify evil actions. To say they are sick is to tell us nothing about them, although it tell us much about the speaker, who seeks comfort and security, not understanding. The obvious danger in this is that proposals to prevent mass murder by the "mentally ill" will lead to greater violations of the civil liberties of people who are no threat to anyone. (Involuntary "medication" and "hospitalization" are common in the land of the free.)
If you want to see what Szasz had to say, you can start with The Medicalization of Everyday Life: Selected Essays, The Manufacture of Madness: A Comparative Study of the Inquisition and the Mental Health Movement, or Psychiatry: The Science of Lies. If you want to get in more deeply, I recommend Insanity: The Idea and Its Consequences, Schizophrenia: The Sacred Symbol of Psychiatry, and the book that started it all, The Myth of Mental Illness: Foundations of a Theory of Personal Conduct.