I liked the little insight that Zeref gave about how Acnologia thinks about humans. It thinks of humans as vermin. That is kind of what we do now a days to justify the killing of others of our own species. For some reason I get the feeling that this episode was kind of making references to the darker side of humans in general. But that depends on if you want to interpret Acnologia representing that. Just remember that It was Hades and his sins that summoned Acnologia. He justified the killing of countless people because he was trying to awaken Zeref and create the Great World of Magic. Even Zeref took offense to that.
I have to say though that Acnologia is one strong creature. Makarov was barely abel to contain Acnologia while Fairy Tail ran away, Not like it mattered because they came to retrieve the master they love so dearly. But not even the combined might of the guild was able to defeat Acnologia. Acnologia was just having fun with Fairy Tail in the end. And it showed that it got annoyed after that unison attack by Fairy Tail. Bye-bye Tenroujima.