What is the driving force behind these extreme reactions, what makes them extreme within their ideology?
Most people will just look at the ideology itself, in this case, this makes all the people who follow such ideologies, that can be expressed in an extreme way, extremists in some way. This analogy would also make all Christians extremist as well, especially when considering the religious Dark Ages. Does the actions of Christian extremists in the Dark Ages infer that all Christians today are extremists? Of course not. Extremism hasn't anything to do with the ideology itself, its to do with the implementation of such ideologies by certain people religiously following an ideology.