User blog comment:Omega natsu2/Chapter 359: Fairies vs. Hell, Predictions./@comment-5426822-20131102154732/@comment-1359322-20131102164905

Well it could also be that they are trying to cover up something. Like Tartaros' weaknesses. Maybe the Council has some tools that can exorcist the Demons easily so they decide to kill them all first. nothing was known about Tartaros till Cobra reveals that they are all Demons from the Book of Zeref, so that's why the Council hasn't taken any action against them.