User blog comment:FairyRaven/Fairy Tail Chapter 414: Drops of Fire, Review and Commentary/@comment-24277053-20150105175551/@comment-7418318-20150105180442

Acnologia came for the Book of END and then switched over to killing Igneel. With the book gone and Ingeel most likely dead, he doesn't have a reason to stick around and can just fly off without it being unlogical or anything... especially considering the one severe and numerous minor wounds that Igneel gave him.

I think if Igneel still has a role in the story, it'll only be as part of Natsu's memories or at most as a disembodied Spirit a bit like Atlas Flame.