Group psychology; all groups, whether national, religions, ethnic etc will follow certain universal and specific norms. Those specific norms are generally exclusionary. Anyone who’s not one of us is automatically inferior. If the universal norm is to be tolerant, then tolerance is an accepted part of the group norms. Unless the value of a group norm (not universal to all groups co-habiting) outweighs the needs of the individual, then you see rising group action against out-group members.
Generally it comes down to positions of power and the benefits they can bestow on your group. If a group ‘feels’ as though they are in danger of losing that privilege then this can easily overcome individual values. Nationalism, ethic and religious conflict all tend to fall into this concept. There’s logic in the madness.
Mix in a fair bit of ideology and blood feuds and you have the world at any point in time.