User blog comment:IamJakuhoRaikoben/Chapter 497: The Winter Mage, Review./@comment-24951162-20160810202218/@comment-1776824-20160816022644

If Natsu had Etherious powers, why would Igneel make him a dragon slayer? Why would Zeref support this idea? Why would Zeref have waited for Natsu to grow stronger? Why send him 400 years into the future and have him gain strength in an era with more ethernanos? The answers are as follows.

Natsu would have been strong enough to kill Zeref from the beginning if he had this "Etherious Form" or "separate personality;" clearly, as we saw from his revival, he was made from the original Natsu's body, as it was clearly stated to have been used as the base for E.N.D., which made him different from the other Etherious, who were made of Ether. Being made of Ether allowed them to change shape, and that leads me to my next point, which is that the "Etherious Form" is their original, true form. Their human shapes were powered down forms they took, mostly so Hiro had a hidden strength to display for them.

What's more, Natsu was the last Etherious. The very last. If you re-read the chapter, Zeref says that he made a bunch of Etherious first (it shows pictures of Tartaros and Deliora), and then it says Natsu was the final creation he made, because the other Etherious couldn't do ass to him. So let me repeat: Natsu is the last created Etherious. What makes Natsu special and different from other Etherious is his ability to grow stronger, whereas the other Etherious all have set, finite amounts of power. Natsu was weak, so Igneel took him in (chose not to kill him because he grew to love Natsu despite knowing how dangerous Zeref's creations were), taught him magic, blasted him into the future, and here we are. Natsu has grown stronger.

And why didn't Igneel want Natsu to open the book? Simply because if Natsu opened it, he would learn that he is an Etherious. That he is manufactured; manmade; however you want to put it. He would learn that he died. That he is Zeref's brother. Igneel wanted to protect Natsu, and as for destroying the book, Natsu would have clearly died if that happened. And Natsu and Zeref were clearly not together for very long after his birth, because Natsu does not remember him at all. He only remembered the time he spent with Igneel.

As far as curses are concerned, I couldn't fucking tell you. E.N.D. came after the other Etherious. They all clearly had these powers beforehand, so the description of what curses are and how they work are accurate, but their actual origins, as far as I am concerned, are a mystery. Mard Geer is not a credible source for anything since he became so disillusioned with everything that even he started to believe the nonsense he was spouting.