My mother was a narcissist who lived to be 98. It took me about 35 years to figure out that she was also a persistent liar (when she was 67). The reason for my slow take was that she was a mimic and when she was around me, she acted just like I did. It was long obvious that she did not like men since she had deserted my father after he returned from four years as a prisoner of war and never had a positive relationship with any other man. It was when she turned on her young grandson for having done someone else a generous deed that I realized she had been lying about my father from the beginning. She acted as if generosity was stealing from her.
It took even longer for me to realize she did not like women either. Anyone who did not satisfy her desires (which were often unknown) was considered an antagonist and subject to be retaliated against. Even at age 95 she threatened to ruin a dentist because he had failed to fulfill a totally unrelated expectation. She even recruited an unwitting acquaintance to help her in secret because she knew I would not approve. What she had told the acquaintance was that I was her hired housekeeper and nurse. That my husband and I were hired hands was apparently a persistent illusion. During the last week of her life when various organs were shutting down she roused from an apparent coma to formally dismiss us as faithful servants. She did not know us; never did.
As Donald Trump blithely goes through life dissing relatives and friends regardless, the pattern is totally familiar. What is to be done? Impose restraints. Maintain him in a secure location and monitor his outside communications. Remember that any unsatisfied desire is considered a theft.