I finished chapter 28 so here are spoilers for those who want them. And an explanation on why killing the chiefs won't work.
Apparently there has only been one case of the demon lord killing a disobedient chief, but that was only once. And it was to get a unanimous vote. In this case, even if the chiefs are killed, it does not change the fact they have voted to oust the demon lord. And once half have been reached, even if the chiefs are dead, the vote stands.