User blog comment:BlackWizardZeref/Fairy Tail Chapter 394: Juvia vs Keith Review/@comment-24930767-20140725110905/@comment-66.86.223.64-20140725205405

Well it seemed more like Igneel couldn't kill him because he chose not to, which would lead us to believe that Natsu is END. And that's why Natsu would have the choice to kill himself just how Igneel had a choice. Also, I think the part about END being in a book could simply allow END to be awakened inside Natsu.