For your destruction is the will of the gods! And I? I am their instrument!
~ Prophet of Truth
And He will smite the wicked and plunge them into a firey pit!
~ Judge Claude Frollo

Fanatics are people who are highly devoted to a religious, political or ideological cause to an oppressive and extremist degree. They are one of the most dangerous characters the protagonist(s) have to face since they believe that everything they do, no matter how horrendous, can somehow be morally justified by their beliefs.

They often play the victim (or, more specifically, the martyr) when their deeds are exposed and brainwash other characters into believing that only their beliefs are correct and scapegoat those who don't agree with their beliefs as being ironically evil and conspiring to destroy or kill them. They are extremely defensive with their beliefs and are willing to commit horrendous acts of violence (including terrorism) in order to preserve their beliefs from "enemies."

In many cases, the fanatic may be a rouge element, not being a good representative of their respective belief and doing things that would normally be considered taboo or wrong by their own belief system, yet finding ways around it by using the same system.