User blog comment:FairyRaven/Fairy Tail Chapter 399: Wings of Despair, Review and Commentary/@comment-24066070-20140908130106/@comment-24066070-20140908152134

Given the fact Acnologia once slaughtered many Dragons before becoming one himself, he likely took or still takes great pride in his role as Dragon Slayer.

If he is proud of being a Dragon Slayer, he might see some purpose in training other Dragon Slayers in the hope they will walk in his footsteps.

Of course, Natsu and the others won't hunt Dragons down like Acno did, but they may just accept the invitation in order to get the strength needed to protect their friends (typical FT reasoning)

As for him regarding people as bugs, it may imply that he hates all people who aren't Dragon Slayers, after all the majority of the humans don't even have Magic, let alone Magic which he could deem equal to his own.