User blog comment:Wrath022/Chapter 516: The Truth About Enchantment, Review/@comment-5303512-20161227040743/@comment-5303512-20161227133022

You see, that is where you are respectfully wrong, Normal Natsu and Etherious Natsu are as different as man and the ape, day and night. Sure normal Natsu wanted to beat Zeref but with reason, to protect his friends, he did not make it his life long goal to defeat Zeref, he did not even know of his existence until Lullaby, or even cared until Zeref turned his scarf black and was going to beat him up for that, not because of his instinct, later would defeat him for the sake of his friends. But when he learned he would die if he killed Zeref, he didn't want to make his friends suffer.

But with END, it is the pure raw instinct that makes Etherious Natsu what he is, and that it totally OC. He did not care about dying, he had no reason, no goal, no purpose, just mindless killer instinct which is not Natsu's personality, something totally deviating from the norm.

It is like Ego and Id of one's mind, or that of Ichigo or Hollow Ichigo.