In the minutes leading up to noon on September 11, 2018, Jared Johns, a former Army private, lay down on his bed, switched on the camera of his iPhone, and expressed his goodbyes to his family.

Approaching the conclusion of the two-minute video, Johns's eyes widened in horror as he read a message on his screen: "She is informing the police, and you will be imprisoned," the message declared.

Post-Afghanistan service, Johns took a deep breath, positioned a 9 mm handgun under his…