You don't have to invoke all of that to commit atrocities, just appeals to convenience work well enough to convince someone to forsake the rights of another person. Settlement of this country should have never happened, this is all stolen indigenous land whose conquest was legally justified by the handwaving of a patriarch of a religion hundreds of years ago. White settlers at the time saw native peoples as a pest much like a coyote is a pest to livestock, and much like coyotes they were allowed to kill off these people pretty matter of factly without recourse. There was no fear or emergency justifying that action, only that the white person would like to make profit grazing their cattle in the buffalo country on land that was offered to them by their government for next to nothing.