I doubt Natsu is a demon since he uses actual magic and not curses like the demons, also Natsu is Zeref's brother or at least the clone of his brother.
I think that in chapter 436 (the one about Zeref's memories) it's said somewhere that Zeref kept his brother child's body in order to bring him back to life (Natsu was inside a glass container or something). So, as Zeref himself, Natsu was human formerly. After Zeref got cursed and started to create 'weapons' that could kill him later he must have made some alterations on Natsu's original body and pretty much accomplished what he wanted: his brother revival if you think about it. Maybe not the way he wanted since Natsu doesn't have any memories of Zeref as his brother and also that he was able to grow up. I think is (not so much) obvious to presume that those memories are inside END's book. THEN, maybe when he brings back his little brother, the 'demonic' part went out of control and that's when Igneel became involved with him, to try stop him and the rest of the story we already know how it goes.
actually, zeref has already revealed to the readers that END stands for Etherious Natsu Dragneel in chapter 416 (=・ω・=)
Yes but not to the characters themselves. Those are two different things
Im 99% sure that Natsu is Zerefs most powerful demon END. The brocolli guy said that Igneel was fighting END but decided not to kill him and that would make sens how Natsu meet Igneel. Also why they never showed us how END looks like. Plus Natsu is Zerefs brother and he created Natsu to kill him and always says that only Natsu can do it. There is probably a seel on Natsus power END.
I wonder what Gray will do when he finds out and Lucy ...
Well Gray will probably want to kill him but finally they will say something about fairy tails bond^^
Im excited to find out if Im right but i hoped there would be some romance between Lucy and Natsu befor the fight with Zeref that would make it more excited and emotionall.
hopefully Im right :)