User blog comment:B214/Chapter 503: The Last Sight Seen/@comment-5303512-20160926122648/@comment-5303512-20160928175059

My mistake, this is the first time awakening since the beginning he was END, but ever since what ever happened back then and to the current chapter, Natsu had been Natsu, not END. So I correct myself, this is the first awakening.

While I do admit that Hiro may not have thought of the demons killing Zeref thing back then, it is still part of the story and is usable in your speculation about 400 years of adapting to behavior and learning.