There are all kinds of things that are not stereotypical mainstream behavior that are not mental illness. For example, some people speed. Going 100 mph down a highway isn't "normal" behavior, but most of the time, it isn't mental illness either. One incident of shoplifting isn't "normal" behavior, but most of the time, it isn't mental illness. Sometimes, criminality is just criminality.
There's a lot of complexity to human behavior and while some abnormal behavior can be caused by mental illness, what people fail to recognize is that some NORMAL behavior can also be due to mental illness. You can't label anything that's a deviation from mainstream as mental illness. That's how we end up with homosexuality being considered a mental illness, which thank goodness, we no longer are. Our personalities, impulses, wants, needs, and actions are all formed and maintained by a number of factors and mental illness doesn't always play a role in it. In fact, I'd say most of the time, it doesn't, IME.