User blog comment:FairyRaven/Fairy Tail Chapter 399: Wings of Despair, Review and Commentary/@comment-24066070-20140908130106/@comment-7418318-20140908143018

@Sky-Dragon Twilight: As a Dragon Acnologia considers Humans as bugs. But if it loses most of it's powers and is returned to being Human, that might no longer be the case.

Here the want for revenge against the Demons who took it's powers away might outweigh whatever feelings of disdain Acnologia has for Humans.

It could even be that Acnologia lost his mind (or part of it anyway) when it became a Dragon and that before the transformation it was "only" rassistic against Dragons and/or obssessed with gaining more power, but didn't hate or dispise Humans. And with Zeref's rumored involvement in the transformation, it might even be that Zeref somehow locked up what little mercy and commpassion were left in the Dragon Slayer.