America’s young male killers, who glory in indiscriminate slaughter, are a Western eruption of a larger phenomenon. What’s often called “stateless terrorism,” or deadly extremism by individuals and groups, could as accurately be described as a male crackup. This convulsion has been most acute in the Muslim Middle East, where modern authoritarianism has bulldozed the past, freeing young men from once-powerful social constraints and exposing them to a radical religious rebellion. America—the relentless engine of innovation and change—now also faces an outbreak of disconnected young men murdering exuberantly.

It...