"Abnormal" normally refers to something that doesn't happen frequently or isn't normal.
Behavior that is abnormal does not follow the rules. Our society defines abnormal behavior as behavior that doesn't meet expectations. It is imperative that people like these are dealt with immediately. This type of behavior is studied in Abnormal Psychology.
This branch of psychology deals with people's unusual behaviors. It is the psychologist's job to determine why and what caused the abnormality, and then determine how to recover from it in the most effective way.
It is necessary for these patients to receive clinical treatment.