xian fundies tend to be this way. It attracts people who have an immature emotional need to be better than everyone else. And they get that need met in fundamentalist settings.
Fundamentalism also disregards the importance of intellect, logic, learning and critical thinking. Possessing those things tends to eliminate egotism.
Once you learn and understand the fact that there is no god and we are all equal products of an indifferent universe, it's more difficult to be narcissistic.
most personality issues are attributed to exposure. If you are surrounded by narcissists, then it is easy for you to become one
Yes, and possibly genetics as well, which is an issue for jws because many are born in or bred from >1 family of jws.