Making a thread NPC that is likely to murder another member's characters can be considered godmodding in extreme cases. Labeling a thread "DE" (death enabled) is a surefire way to protect yourself from most cases. A member can also give you explicit permission to kill their characters. Accidents do happen, and the staff is understanding of that, but we don't want to deal with people being unhappy if they died because you want to force them to fight a mega-boss.
Moral of the story: be mindful of what people in your threads can handle, and what they're willing to do. If everyone else in a thread doesn't want to face a giant pig rat dog changeling with 100million PL, then they have a right to ask you to change things.
Repeated cases of abusing thread NPCs can be punished with IC punishments Or OOC punishments. Commonly involving persons permission to play NPCs Revoked.