Have you ever, walking down the street, turned instinctively at the sound of something a little “off,” abashed to discover a person with a limp approaching? It doesn’t mean you’re cruel, intending to cause embarrassment. I wouldn’t stare at such a person walking directly towards me. What I’m describing is instinct picking up something that might indicate danger. Tens of thousands of years went into our ancestors being alert to anything different. Few sabretooths today, but we still spin at the sound of an irregular footstep approaching.
Humankind—those that turned and looked and survived—absorbed these lessons long before they had doctors and psychoanalysts to diagnose. It’s still a good rule of thumb to determine what danger a given action or individual might pose. If eating a red berry from a certain bush meant illness or death, the members of your group stopped eating those berries, warned their children, “Danger! Don’t eat!”
If you observe a neighborhood child frequently bullying playmates and tormenting animals, you don’t need a medical diagnosis from the DSM V of a specific mental illness to determine that that child poses a problem for the local kids and pets. He’s dangerous.
This rule of thumb works for adults, and covers social and work areas as well. You don’t need refinements and definitions. Mental illness does not, in and of itself, indicate danger. Many, if not most, such individuals can work and pursue life without harming others. Many human traits are shared along a spectrum and may be benign or even helpful. Narcissism, to take but one, is a personality component for many of us, an asset even, especially for those who perform, create, or put themselves in some way into the public view.
The thing to assess is what traits a given person exhibits, how many there are, to what degree they are displayed, and how often. Did the little boy next door accidentally hit your daughter with a rock and was horrified when he cut her cheek, or does he attack children daily with an intent to harm?
Observe, assess, then act to remedy, would be my advice. When to act, and how exactly to cause the problem to stop is the question you need to decide for yourself.