This is my explanation for what happened to Natsu
When a human is given the power of a Dragon (Dragon Slayer Magic), that power (Dragon Seed) slowly takes over the body and the more you tap into that power the more you forget that you're human and the more the seed grows till you lose all humanity. Such as how Irene didn't fully become a Dragon until she lost her faith in humanity after being tortured for so long. Shredding herself of that is what finally caused the transformation.
The same goes to the power of Demons, you just need to look at Natsu and Gray. When got consumed by rage after thinking they lost the person they loved most (Juvia/Lucy) they both tapped into their inner darkness Natsu (E.N.D) and Grey (Devil Slayer Magic) they started to lose their humanity which Ivel touched on in his fight with Gray and the power they was using was starting to take control. When Makarov sacrificed himself and they see Erza crying they regained their humanity which is why they returned to normal.
In Natu's case, it was a bit different because the Demon Power that had started to take over his body had begun to fuse with his Dragon powers.
As for what happened at the end was explained when Lucy said to Natsu he will die if he kills Zeref, to which Natsu said lots of things happened inside my heart and I was told to believe I am human and Lucy said such..... such reasoning.
Basically meaning that E.N.D and the Dragon Slayer Magic is still within Natsu but as long as he remembers he is human and does not lose his humanity that power can never consume him.