User blog comment:Derax-Pagani/Chapter 507: Voice/@comment-4449950-20161028023156/@comment-82.168.3.87-20161028134816

The biggest hurdle to this theory is that when Erza and Erza fought, the animosity they had for each other was cleared up after the fight. When they split up at the and of the Edolas Arc, they where no longer enemies (although they where not exactly friends either. So there is now reasonable explanation for why Eileen has this intense hatred for Erza if Eileen is Knightwalker. Indeed the 7 year gap explains their age difference but it doesn't explain the animosity that Eileen has for Erza. This goes for the Future Erza theory as well. The wish of Eileen to kill Erza is not explainable within these two theories in my opinion