The thing to keep in mind is that there were hundreds of Native American societies, and they varied considerably in their social practices. Slavery was fairly common on the Plains. The Pawnee still practiced human sacrifice in historic times. The Beaver Wars were unbelievably genocidal. It may be the case that some of the Siouan groups (such as the Quapaw) moved west was to escape social collapse and genocide. The enormous diversity of NAtive societies has been largely forgotten/erased through romanticization (itself a form of cultural domination) and sociocide).