Last weekend’s mass shootings in El Paso and Dayton accentuated a painful fact of 21st century life: the lurking fear that a lethal threat could erupt at any public gathering. Anxiety is soaring among the faithful (domestic terrorists have attacked houses of worship), shoppers (malls have been shot up), Hispanics (targeted in El Paso), concertgoers (remember the Las Vegas massacre?), parents and teachers (school shootings have come a long, lethal way since Columbine in 1999). Everywhere, it seems, there are signs of a society under siege.