Psychologists find that agreeing on the differences between typical and abnormal behaviors is surprisingly difficult. however, deciding that an individual’s behaviors and mental processes are abnormal is essential because

However, deciding that an individual’s behaviors and mental processes are abnormal is essential because:  it is the first step toward labeling that person with a psychological disorder and providing treatment
For example, someone that could kill someone without remorse are considered a mentally ill person if they're in normal society, but that behavior is really acceptable if that person fights in a battlefield