User blog comment:Renkrisb/The Dragneel Brothers/@comment-26416024-20150519215248/@comment-1776824-20150520040359

Yeah, Zeref said that, but he also then said, "Life or death? The choice is yours," and before that remarked that, "Soon, you'll be faced with the very same decision."

Igneel most definitely chose not to kill E.N.D. and our own translator verified this. And whatever reason that Igneel may have had not to kill Natsu has to do with how Igneel found him over 400 years ago, and what their relationship at the time was. I am almost certain the wall painting of the figure fighting that Dragon in Crocus was most certainly E.N.D. fighting Igneel. However, I am pretty sure that Acnologia fears nothing, not even E.N.D., and if anything, based on this last chapter, he would relish the thought of fighting E.N.D., because Zeref hinted that Acnologia lost his arm due to carelessness and not taking Igneel seriously.

What I won't deny, however, is that Natsu's memories and powers as E.N.D. are sealed in that book, because at this point it's extremely obvious that that's the case. I am almost certain, though, that Igneel was the one that did this (somehow), and that that was his alternative to killing Natsu.