A grocery store. A hair salon. A church. In less than a week, these everyday settings have become the tragic backdrops for the latest outbreak of what has become the quintessential American expression of deadly violence — a mass shooting staged by a lone gunman armed to the teeth with guns and grievances. Experts who have studied the phenomenon said the three shootings, which have left 11 people dead and 11 more wounded since Wednesday, underscore a sad fact that most of us already understand on some level — that no place is safe anymore. “They are targeting people in...