User blog comment:Iron.Shadow.Dragon/Nine Demon Gates Match-up Predictions/@comment-5303512-20131012152446/@comment-7418318-20131012215113

Each of the members of the Seven Kin of Purgatory wanted to bring about the "Grand Magic World" (or whatever the exact name was)... except Meredy I guess, she just wanted to help Ultear.

Whether they serve Zeref with Hades as his proxy or they serve Zeref directly won't make much of a difference to them, as long as they believe that Zeref intends to turn the world into their "utopia".

Or if they realize that their perfect magic world was just a misguided dream, they might follow Zeref for other reasons (like sticking with the strongest guy for example).

So while there are reasons for them to part company (on the assumption that they weren't killed by Zeref, whether intentionally or by accident) there would also be more than enough justification to have them be Zeref disciples now.