In the New Zealand case, the perpetrator’s manifesto laid bare his extremist beliefs. But the gory video is what demonstrates the consequences of that ideology.

When white supremacy is thought to live only in words, it becomes abstract, mutable, even deniable. The very real extensions of that ideology are easy to ignore.

It’s in this context that the importance of excruciating documentary films like Alain Resnais’ 1956 “Night and Fog,” a 32-minute film about the mass murder of Jews during the Holocaust, becomes apparent. The French government periodically re-airs “Night and Fog” as a public service: to remind people of the horrors of war. Even some American high schools in the ’80s used to show it to make the devastation wrought by racists with weapons indelible in the hippocampus.