User blog comment:FairyRaven/Fairy Tail Chapter 301: The Dragon King, Review and Commentary/@comment-4933386-20120929060029/@comment-4388213-20120929104817

I've been thinking that.

Maybe Zeref accidentally caused Acnologia to become what he was to day, they both were probably DS back during the war and Acnologia became power hungry seeing that Zeref tried and failed to stop him leading to him to become a dragon and attacking all (humans and dragons). He probably did something so Acnologia wouldn't appear unless the levels of malice are high enough or his presence is known.

Scared and ashamed of what Acnologia did and out of fear that another human would do the same created the Eclipse plan causing dragons and humans to become separated. Whilst attacking any humans who don't value lives to insure that Acnologia doesn't appear and constantly reminding humanity of their mortality.

But thats just my hunch