Why?

I often feel as if I cannot articulate something better than I have already read it done by others. Once again, I find myself in that state:

What is it about the human condition that induces otherwise ordinary people to murder the powerless, whether human or nonhuman, with such evident pleasure? For it is indeed pleasure we see in faces of whites celebrating beside the sexually mutilated corpse of a hanged and burned black man in the American South, pleasure that onlookers saw in the animated faces of Hutu men and women as they swung machetes against their defenseless Tutsi neighbors, singing, pleasure etched in the smiles of Gestapo officers laughing as they kicked naked Jewish women cowering in the dust at their feet.
~John Sanbonmatsu

I have seen all the images referenced here, and more. Yet, I continue to be dumbstruck by the depths of our capacity for depravity and cruelty.