If another Dalmite attempted to usurp the position of another (Say: clan leader, head shaman type positions), would it be legal to kill them for their attempted mutiny?
Yes, it would be acceptable. It would be considered treason to the clan leader or other leader and the death would be justified. There would probably be an inquiry though by the tribe or clan council. The only exception would be a legitimate challenge to the leadership, which is usually fought through a wrestling type match. However, challenges of this type are rare and often, the challenger ends up dead by mysterious means before the match can be held, especially if the current leader is loved by her people.
