Each massacre is a bit harder to fathom than the one before.

High school students. Elementary school children. College students in Blacksburg. Sikh temple worshippers. Movie-goers at the Batman premiere. The worshippers at a Wednesday night prayer meeting.

The list in truth is much, much longer, written in red across the pages of American history in the past two decades. Each massacre has been a deliberate act, some of them, it seems, perpetrated by persons directed by uncontrollable inner voices, others by zealots acting purely out of hate.

Other than the easy access to guns, other than the nation’s patchwork and often poor care of the mentally ill, these massacres also have a very bad thing to say about the state of the national discourse. 

For that discourse, and no one can deny it, is filled with vitriol and division. Partisans of this cause or that cause often feel the best way to gain a point is by taking no prisoners. They demonize all who hold another view. Voice...