Taking a somewhat opposite tack here, I will say from the research I've done and the counseling I've had, people who are afflicted with Cluster B personality disorders (Narcissistic, Borderline, Histrionic and Antisocial/Sociopath) are very difficult to treat.

In the words of Colleen E. Warner, Psy.D., even therapists are loathe to treat them due to the high emotional cost to the therapist:

"I was advised by the malpractive folks to refer all borderlines to my worst enemies."

~~ Borderline Personality Disorder, Struggling, Understanding, Succeeding, page 2
I think it may be very important to understand if one is dealing with a disordered person, because dealing with them is bout as successful as dealing with an active alcoholic or addict, as ML states.

Cluster B personality disorders are learned behaviors, caused by traumatic and dysfunctional experiences during childhood. They are not based on genetics or chemical imbalances.

Preach the Gospel every day. When necessary, use words.
St. Francis of Assissi