Edit: Cleared up next chapter...
For some reason they think that bad things happen when dungeons are cleared. Amunzars is not trying to clear dungeons so they're not "a danger to the world". They are trying to kill those (usually parties) who have the potential to. Orn can potentially clear the dungeon on his own which in their eyes make him "a danger to the world".
Also she's probably being used as a pawn. We don't know the true intentions of those behind this, but probably once they attain their goal she'll be taken care of.
The "cult" could be the organization directing the Amunzars or it could be that the "cult" is actually being directed by the the Grand Master (ch 17). Or it could be that the Grand Master is directing the "cult" that is directing the Amunzars. We don't have enough information to know. The Light Novel readers may know.
