The author of the landmark work On Killing reveals how violent video games have ushered in a new era of mass homicide--and what we must do about it. Paducah, Kentucky, 1997: a 14-year-old boy shoots eight students in a prayer circle at his school. Littleton, Colorado, 1999: two high school seniors kill a teacher, twelve other students, and then themselves. Utoya, Norway, 2011: a political extremist shoots and kills sixty-nine participants in a youth...