I think every adult converted to JW cult is a Npd or have some temporary psychological disease.
Npd is a permanent psychiatric disease. It's something related to neural connections.
The diagnosis of Npd is almost useless to the person itself but extremely necessary to people living with a person who have a Npd. The only thing one can do to protect itself from people with npd is avoid them like the plague. If you cannot avoid them completely you must minimize the contact at least.