User blog comment:Vitalius/The return of Oración Seis and Crime Sorcière/@comment-24066070-20131023130053/@comment-24066070-20131023135420

It is probably true that Tartarus considers OS too weak, as FTGM said. But Tartarus has recently started gathering forces, as seen when that woman turned the members of Minerva's new guild into paper figures (they looked like paper figures, not sure if that's what they turned into). So one would think "the more, the merrier", so they could just free OS while they're attacking the Council anyway and turn the OS members into subordinates.

@B214: Jellal said the purpose of their guild is to destroy the evil created by dark guilds and Zeref. They also stalked the Zeref-like magic that appeared at the GMG in order to destroy it. So I don't think it's extreme to state they wanna destroy everything related to Zeref.