User blog comment:FairyRaven/Fairy Tail Chapter 416: Tartarus Arc: Final Part, Review and Commentary/@comment-24367018-20150119232736/@comment-9595671-20150120030651

Its possible its his super powered evil side, Igneel never defeated END but no one ever said its because he lost. He likely won, then END turned into Natsu, Igneel suddenly felt pity seeing him as a young child and decided to raise him as a son.

Here's where I'm getting curious, due to Igneel raising him what happened to his END powers? Is it possible they've been erased? While maybe the evil twin thing is a stretch, I don't think its a stretch if Natsu's old powers can't be returned and they get remade into a counterpart, that fits better as Zeref created living magic.