Every human being on this planet has both light and darkness within them. All beings in nature have a light and a dark side. It is the nature of existence.
That light and darkness is expressed in many different ways in the varying life forms on our planet. We as humans seem to be the only creatures which are out of balance with this natural duality. We seek to suppress the darker sides of our nature. We seek to deny that it is there, believing instead that it is a sinister evil that much be destroyed. This only forces them to the surface in sudden violent ways, much like a volcano erupting.
To be truly happy, to be truly free, we must accept and embrace all parts of ourselves, the good and the bad, the light and the dark. When we do that we will find we come whole.