User blog comment:IamJakuhoRaikoben/Chapter 497: The Winter Mage, Review./@comment-24951162-20160810202218/@comment-2214433-20160811132632

Zeref literally said that Mard Geer picked up END's book and made shit up in order to band the demons together as Tartaros; he claimed END to be their master, the creator of curses, and used the book as a common goal of Tartaros: to 'revive' END in order to return to Zeref.

All of those were lies. Zeref has said so. Perhaps Mashima just changed his plans by turning Mard Geer into a liar, maybe he just had no time to show END's past, but this is the route he chose.

Igneel indeed trained Natsu in order to defeat Acnologia, however, Zeref agreed with that only because he hoped Natsu would kill him if he became strong enough, Zeref didn't really care about Acnologia himself.

Natsu having any demon form would be contradicting all of the above.