The big mystery is, that even total "normal" people can become "beasts".

The problem is, that we are so far away from all that, that we think, we are immune to such things ... but we are not. Because humans are essentially the same today as yesterday. The only thing we could/should do is, remember what happened in the past. But we do the opposite: We make unnatural beasts of those people, that existed in the past -- some fairy-tale like creatures, that existed only in a nightmare of humanity.

But it was no nightmare and it can come back to us very quickly. Look into the nearer past -- what happened even in the "peaceful" Europe (e.g. Yugoslavia) -- humans can easily be beasts again and much easier, when we think, that it all was a fairy-tale.