User blog comment:CNBA3/Tartarus' Army!/@comment-46.198.201.133-20131105182205/@comment-7418318-20131105213421

Come to think of it, there are "free" Demons living on Galuna Island.

Could it be that the Nine Demon Gates weren't created by Zeref, but that he caught them and bound their souls to the books, thus enslaving them?

Like that E.N.D. might be something like a master control, ensuring that they all follow the mindset that Zeref gave them at the moment of their binding 400 years before.