User blog comment:IamJakuhoRaikoben/Chapter 357: The Nine Demon Gates, Review./@comment-5580383-20131025195248/@comment-1359322-20131026022242

Maybe they're just doing it to seal off the Etherion. Since the Magic Council are the only ones that have the access to it. And now that they killed off the current members of the Council, there's a possibility that the old members will be reinserted back, so they decide to kill then too.