Perhaps it stems back to the split that changed the skinwalkers from respected tribe member to a position of outcast. If the skinwalkers were originally of a great help in locating food and enemies, and with the understanding of how the tribe is usually of such closeness, as to be an extended family, then a purposeful deception/betrayal that led to an attack on the tribe would be seen as attacking and killing of a family member would it not?

Perhaps this betrayal of the tribe/family is what has caused this cutting of ties to the skinwalkers from their own tribe. So the animosity has fueled the fire so to speak! As Native American cultures place great value on life and family, and the skinwalker's MO is to harass and attack tribe members/family then this would be a "black"/evil art that could be seen as killing of a family member(causing harm to a tribe member)! At least to my limited understanding!

Regards,
Lobo-hotei
lobo

Treat the earth well, It was not given to you by your parents, It was loaned to you by your children.
Native American Proverb