Moral rigidity and its intimate link to in-group boundaries may have evolved so as to make us behave, and be seen, as trustworthy yet cautious team members in social environments mired by intergroup competition.
Radicalization is an inherently relational concept. One can only be radical in relation to someone who is not. Similarly, one cannot be extreme without an accepted center norm. But the center is not a fixed state. It shifts and changes across time, place, circumstance, and culture.