What violence lies in a heart so troubled that the actual and bloody injury, dismemberment, and killing of others is in any way satisfying? 
I suppose every heart has been angry and momentarily wished violence upon those who have wronged it. 
Depending on the person, such feelings are acted out on a limited scale, and a festering feud ensues. Sometimes they are subdued and harbored for long periods of time without their being enacted. They may linger for a time, but subside with meditation upon their potentialities, in the passage of time. In other cases, they are abandoned in favor of forgiveness and hope for reconciliation or, if nothing else, a conscious separation from those who have inflicted hurt upon the person.
But how is it that a person -- the breathing, walking, talking, former freight of a woman's womb, who entered life entirely dependent upon others -- actually carries those awful fantasies to fruition, with calculation and malevolence, upon persons both legion and unknown? This is beyond my ability -- or even my desire -- to imagine.
Yet I must ponder it:
Whether due to a congenital malformation or the result of traumatic experience, whatever causes malevolence to be the natural product of a human soul, is a tragedy larger than that single life, because no human life is isolated from others.  
How can we prevent this violence?  We must be vigilant in every possible way, from considering the words which issue from our mouths, to considering how we respond to those wrongs done to us; from deciding whom we elect to represent us, to being mindful of the mental and emotional states of our family, friends, and neighbors, and even the stranger we encounter on the street.