Part of the definition of narcissism is that a person with this disorder is not crazy, and is in touch with reality. But some people operate under the impression that those with malignant personalities are not responsible for their actions, since they are devoid of empathy. Narcissists are also not troubled with feelings of remorse when they break up someone's friendships or run someone out of a job they had depended upon to pay the bills.
However, I don't believe that an otherwise well-functioning adult can't control herself, because once we hit adulthood, we are responsible for our actions. If a narcissist didn't know that what she was doing was not socially acceptable, or wrong, she wouldn't make so much of an effort to cover up her misdeeds. Nor would she be able to spend so much energy plotting to destroy her enemy. She knows her actions are bad, but decides to do them anyway. Narcissists also seem to enjoy being sneaky, and pulling one over on people. This allows them to live under the illusion that they are smarter than everyone else.
Despite my own horrifying encounter with a female malignant narcissist, whom attempted to destroy every aspect of my life, while posing as a "friend," I truly feel sorry for people with this moral sickness. They aren't happy campers, because their lifestyle breeds misery, which only comes back to haunt them in the end. That's because malignant narcissists will eventually see justice.