The darkness in mankind often manifests itself in a hatred of the weak and the different. Human history overflows with oppression, genocide and slavery. Our past is full of mass murder and repression. No excuse seems too minor when a group of people is too different and too weak to defend themselves.
Just look at how humanity has treated the Jews.
Why have we hated them so much?
It makes no sense, but that shouldn’t keep us from trying to understand why humanity is so full of such awful darkness.