Well as far as we know , Natsu is zerefs brother revived with a forbidden curse , and i think that we can say that inside natsu there is something he is not aware of , doesnt matter how hard he is beaten up he doesnt die and his body stays intact, What shows that he is most likely indeed a demon. I think that the cause of natsu's unawareness is because end (the book) hasnt openend yet and so the etherious power inside natsu is present and protects him while he himself doesnt have to do a thing . Also i think that as soon as end awakens he Will take over natsu's body and i  that that was What zeref meant by "Will it be you or end to kill me"  so when end awakens and takes over nastu, natsu could defeat him with his willpower or he succumbs to him What will probably be the end of the world they know and all his guilmates Will be killed with natsu's hands or something... just a Guess tho :3