iTunes |In the pre-dawn hours of July 20, a heavily armed 24 year-old doctoral student named James Eagan Holmes entered an Aurora, Coloroda theater and started firing during a midnight screening of The Dark Knight Rises. In a matter of minutes, a dozen men, women and children were dead, and scores more wounded. Holmes turned himself over to police, reportedly telling them that he was Batman's nemesis The Joker. It was an incomprehensible crime, but only on in a long line of mass murders committed in the United States. What drives people to do such a thing? Troubled by this perhaps unanswerable question, I called Emil Chiaberi, the director, writer and producer of a gripping new documentary on the phenomenon of mass murders: Murder by Proxy: How America Went Postal.