[immblueversion]

The way I see it, Natsu was meant to be a human vessel the demon was supposed to inhabit. Kind of like how Silver was pretending to be Deliora in a human body. The being that is "E.N.D." is still trapped inside his book, unable to escape due to all the magic power in Earth-land, and Igneel raised Natsu to have a will of his own and resist E.N.D. Another guess is that E.N.D. wouldn't need Natsu's body if Tartaros succeeded in getting rid of all magic on the continent, which is why Mard Geer was willing to risk killing Natsu when they were so close to achieving their goal.