I do not think that being fixed in one's ways or feeling disgust for all opponents is a mental illness at all. This whole idea of growing as a person and being open to other ideas and ways of thinking is relatively new, and not being comfortable with change, or being so sure in your beliefs that they cannot be shook or swayed is not a disorder. And as far as wanting to eliminate opposing views or having strong negative feelings towards opponents, not liking certain groups of people because of their views and beliefs may make you a jerk, but it isn't a mental illness.
What you are saying is true in general. What these really are is just a matter of being developmentally immature. At a certain stage of growth these narcissistic behaviors are in fact part of normal development. Everyone grows through them in early childhood where they are initially incapable of taking on the perspective of the other, to empathize with them, seeing only their point of view as right and true. Then as they mature they become able to move out of themselves as center and take on the other's point of view, putting themselves in their seat and seeing through their eyes in a 2nd person perspective (a perspective of a perspective). Then as they continue to mature they move into 3rd person perspectives (a perspective of a perspective of a perspective), then 4th, 5th, 6th, and even higher perspectives beyond those in truly advanced cases. Most mature people however should be able to at the very least take a 2nd person perspective, if not 3rd or 4th person perspectives to be considered healthy adults.
So why then is fundamentalism a disease? Because rather than becoming a supportive structure which promotes healthy and normal growth through these stages of development (mentally, emotionally, spiritually), it turns in on itself,
absolutizing its current perspective, it's own developmental stage and keeps people locked in itself through fear and threats. Anytime the body is constrained in this way, like wrapping bindings around a young developing foot for instance, it becomes diseased, twisted and distorted never becoming fully
functional. It makes the foot, in this example, dysfunctional. Fundamentalism is a structure of thoughts and beliefs which systematically targets anything that might mean the individual may grow beyond the structure itself (which is normal in healthy stages of development), and puts itself as the thing in and of itself. This is what a cancer cell does. It takes over the body feeding itself, rather than supporting the individual's growth and health. It's like the scaffolding on a building which entombs the building itself and doesn't allow any new floors to be added. This is no longer a functional support structure, but a dysfunctional one. It doesn't serve development, it cripples it, overtaking the building itself.
When the body does not grow properly, it becomes diseased. Psychologically, a failure to integrate early stages of growth into higher one creates pathologies. Repression, sub-personalities, and so forth result, and it takes years of therapy to help the individual recover. Fundamentalism in fact does create a great deal of damage in individuals for this very reason. If it is imposed upon them they may end up spending years in recovery, trying to reclaim and rediscover what was taken from them, what was denied them in their development. I've been part of several groups which exist for the recovery of individuals from these systems, and it seriously is a form of systematic abuse. It should be called a disease, as people do suffer ill-effects from it and often need to go through some form of therapy as a result of it. This is not just another form of religious beliefs, but something dysfunctional and damaging.
A good healthy religion on the other hand likewise is a structure, but one which ideally helps the individual to grow through early to later more mature stages of growth. When it is functioning, it grows the person from narcissistic early stages, through the more mature, more inclusive stages, where the person even becomes compassion itself in the world, where they have moved from pure egocentric, to world-centric and even cosmo-centric self identification where they identify with, see as all life itself. Fundamentalism again, is when the structure fails to support development of the person as a whole, halting growth which leads to distortions and pathologies.
