Everyone he knew was killed by the dragons.
I heard most of his back story is going to be covered in the Dragon Cry movie
I think it's something like this: Everyone he knew was killed by the dragons. So he went on a rampage, swearing to kill every last one of the dragon race. Though with time his hatred and rage warped and turned into 'I just want to destroy everything'.
Ah, thanks for the insight, people. I couldn't remember if there was a backstory chapter I missed. Cuz I was like, why's this guy wanna fuck people up so badly?!
...was Acnologia's beef? He just wanted destruction? That's it? No other back story other than that? No why? I don't remember his motivation...