User blog comment:FairyRaven/Fairy Tail Chapter 400: Wings of Hope, Review and Commentary/@comment-9595671-20140913110701/@comment-24.90.187.181-20140913173847

I think we have to put more elements together here than we think. Lets take Sting and Rogue's word that they did kill their dragons, maybe even got their blood in them, that could be exactly the same thing that happened to Acnologia. This could explain everything about Sting and Rogue. They got their Dragon Force because they bathed in their dragons blood. Rogue could have evil in him because of the same process like how Acnologia might have gone bad after becoming a dragon. So now we could see going forward with Natsu, Gajeel and Wendy having to make the same choice. Losing their dragons, but bathing in their blood to get their full dragon force, however, risking turning bad like Acnologia and Future Rogue.