User blog comment:FairyRaven/Fairy Tail Chapter 363: The Stories That Demons Read, Review and Commentary/@comment-112.133.212.13-20131206131123/@comment-7418318-20131206145411

But slightly modifyed it might explain how Igneel ended up raising Natsu.

While he failed to kill E.N.D. he somehow put it in a coma or sealed it and feeling responsible for the poor led who he robbed of his caretakes, he ened up taking Natsu in to raise him as his own son.

Of course for that E.N.D. would have to either have a split personality with a caring, mothly side and a Demon side or be a good being that could be used for unspeakable evils if she got captured by the wrong people (Zeref), but having a good heart, Igneel couldn't bring himself to kill her just because other's might misuse her powers and sealed her up instead, promising to take care of her son.