The human capacity for compassion and wisdom is in stark contrast to our capacity for cruelty and destruction. It is difficult to maintain an optimistic view of human nature in the face of the Holocaust, Cambodia, Rwanda, Abu Ghreib, torture, murder and rape. The only explanation that I find helpful for the human capacity for evil is the Jungian idea that we project our shadow selves onto others, and seek to destroy the shadow side by destroying the other. If we accepted our shadow side and sought to integrate it into consciousness, we would not persecute others, regard them as less human, and seek to destroy them.
But where did the shadow come from? Initially it may have emerged as a defence mechanism, or a by-product of the emergence of consciousness. This is suggested by the myth of the Garden of Eden, when the serpent reveals the distinction between good and evil to Adam and Eve, and then Yahweh says that the woman shall crush the serpent beneath her heel. If the knowledge of good and evil is equated with consciousness, and what is allowed into the light of consciousness is regarded as good, then the serpent (which represents the shadow and the unconscious) must be crushed in order to retain a sense of the self as good.
We can break out of this vicious circle by embracing the shadow, and taming the beast rather than seeking to destroy it.