User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-29874610-20170204151505/@comment-5303512-20170204212344

That Demon Seed did not appear until a few chapters ago, and even then with everything with END, that demon essence did not show until Natsu got "cancer" and that was after Zeref Pierced his book, so Natsu had to have no Demon Seed until after Zeref pierced the book, I mean, the evidence is beginning of the serious when Natsu entered the temple of Galuna Island and was not repelled like the other demons were (and don't say Hiro did not think ahead)

There is also this whole situation between the choices that Igneel and Zeref presented, they are totally different, Zeref said will you kill END or let it live, Igneel said will you be a demon or a dragon, even Igneel did not say anything about the demon or dragon seed being destroyed, though it appears that they crumbled away but isn't it odd that Igneel did not say anythingand?

Even so if Natsu was END the entire time then it still would not make any sense for Natsu to be still alive since there was no confirmed antibodies to stop the growth of the seed, though Igneel did say that it is proof you are END, but that still does not change that he should have died along with the seed since he is END.