User blog comment:Derax-Pagani/FT Wiki Poll Bonanza Part 2/@comment-24546844-20160703004814/@comment-4829046-20160710145106

It just takes one flashback backstory for Mashima to do it. Admittedly it is emotional, but still cheap because you could do that for any villain ever, like literally ever. Even some entity born from evil and hatred has the excuse of originating due to influence of other factors like feelings of mortals or divine decree or what have you.

In RAVE, they just died. A few big ones got a backstory to make their deaths have more impact and relevant to the worldbuilding, but they too died. The ones that survived had more to their stories than just becoming fodder in comparison to the mains.

Even with Acno, you just need to show some poor kid, in a town desolated by dragons, swearing to eradicate the reptilian monsters with the same prejudice as the ones that killed his people, and voila, you have the basis for a vengeance story that when coupled with an insanity card culminated in a savage and brutal individual caring for nothing other than to destroy anything that can oppose him(at least going by current plot). I agree with FT not forgiving him for all the things he has done to them and the world, but Mashima is more than capable of making Natsu give a speech about the love shown by Dragons to humans and thus make Acno regret letting himself get obsessed with destroying them and their ways of life.

That's just the most obvious scenario. There are more creative ways to do it but you get the gist.