It is a very interesting theme but do you really want to start it here? The answer has a lot to do with biological basics of the humans and even more with social, ideological, religious and historical fundamentals and developments. This is no easy stuff and could lead to many disaccords. The political correctness may prohibit to point to some reasons, some communities, some persons.
To give a very crude and basic answer from my point of view: Many people have problems with their own personality and need someone to segregate to feel better. The same is true for societies, religious communities and states. Sometimes special interests of special groups (e. g. the males, the rich, the poor, the leaders, the army, the traders, the believers, the teachers, the priests, the x-people, ...) are better supported by oppressing others.
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