The idea that a person might suddenly snap and slaughter large numbers of people has become part of our cultural understanding through events such as the Columbine High School massacre and the D.C. Sniper case. But this image of the sudden turn from ordinary citizen, quiet neighbor, or non-descript teenager to mass murderer is generally false. People who go out one day and kill innocent people or supposed enemies have typically thought about it, planned it, and even mentioned it to others before they actually do it―and the act is usually the result of a buildup of stress and frustration. Here, Katherine Ramsland, a seasoned crime writer, examines true stories of mass murder and reveals the complexity behind the development of a mass killer. She provides common signals, based on actual cases, that will help readers identify warning signs and understand the various psychological factors that may lead a person to kill. Since the first recorded U.S. case of mass murder in 1949, massacres have been increasing each decade, with workplace violence taking the lead as the most common form. The psychology of the killers, however, differs from that of spree, serial, or situational murderers. The red flags of a developing mass killer are obvious and predictable, Ramsland argues, and people who learn to recognize them may be able to defuse a potentially violent situation before it occurs. Using details from various cases, the author examines the different kinds of mass murders, from visionary to family to workplace, and the distinct psychological dynamics of the different types of murders. This essential book exposes the inner world of mass murderers and dismantles the stereotypes we hold about them.