User blog comment:IamJakuhoRaikoben/Chapter 415: And It Shall Become Your Will To Live, Review./@comment-7418318-20150112233703/@comment-9595671-20150115011034

Acnologia does seem to not be fully sane right now, in fact I wonder if the reason he went psychotic and crazy was because that already happened 400 years ago when he was still human.

From what I think I remember, Acnologia didn't immediately start killing the pro-humanity dragons at first, he was targeting pro-eating dragons. If his family was killed, possible even by accident from a pro-humanity dragon... well that explains a lot why he went psycho crazy and just started to kill them all.