User blog comment:IamJakuhoRaikoben/Chapter 357: The Nine Demon Gates, Review./@comment-5580383-20131025195248/@comment-7418318-20131026080207

The council did have access to restricted knowledge and juding from the comments during the Lullaby incident might even have a copy of the (or one of the?) book of Zeref.

If the council potentially has knowledge that could be of great harm to Tartaros, it'd only make sense for them to hunt down anyone who might have read it, to make sure it keeps out of circulation and thus can't harm them.