@me474
You're right stating that some events of the past can be painted in black and white while looking at retrospective (to make things simple, let's omit the fact that what's wrong and right is decided by the ethical standards of the current age). Slavery is bad. Freedom is good.
But it's just the way our brain making things easier to label for us. In reality, living in the past while being influenced by events, there is no way to label things this clearly (except for being subjected to work of propaganda).
But this manga is doing exactly that. Events depicted in it are not something retold to us by some history teacher or bard (that labeled it for us in black and white). It's a clear depiction of humans being bad and beast folks being good by the author himself! Just look at greedy human faces and look at how cute these beast folks are. The author was lazy and inexperienced to project some popular labels to the actual story. We (readers) should make our evaluation. The author should never do it for us.