If the tribe all agree that "grab all you can and devil take the hindmost" is the right way to do things, then that is a part of the morality of that tribe... However much another tribe (or you, or Zander) may look on aghast.

Morality shifts and changes with history, as people's understanding of what is real and what is important and how they interact changes. To a medieval European, it was immoral to let a witch live; to a modern European, it's immoral *no*t to let them live. Both could give you strong arguments as to why; The main difference is simply that we (generally) no longer believe the witch's actions or existence will do any significant harm.