There has been no real success in treating people with NPD. Those who commit the most heinous offenses are frequently repeat offenders (rapists, pedophiles, serial killers). There is no magic medicine to cure a personality disorder, and talk therapy isn’t the answer with predators.
As a law enforcement officer you will deal with more than your share of people with NPD. From the know-it-all, “I don’t take paper” beat partner; to the command climber who will steal your ideas while belittling you on your evaluation. From the DV suspect who claims she deserved it; to the traffic stop with the guy telling you to catch a real criminal; to the male shooter holding hostages after a dispute with his employer. Recognizing malignant narcissistic traits will help allow you to prepare yourself accordingly. Malignant narcissists should be considered as potentially dangerous.
If you are married to a malignant narcissist, I send my condolences.