Twelve days before Christmas, 18-year-old Karl Pierson stormed into Colorado’s Arapahoe High School armed with a shotgun, three Molotov cocktails in his backpack and a machete. The senior athlete and competitive debater fired his weapon for 80 seconds, critically wounding a fellow student before turning the gun on himself.

In explaining his murderous rage, several classmates cited the boy’s political radicalism: he identified himself as a “Communist” and once wore a shirt decorated with the letters “U.S.S.R.” This identification with a brutal, mass-murdering regime should have been a warning—school officials surely would have worried over a kid calling himself a Nazi and wearing a swastika shirt.

Press accounts mostly emphasize the weapon he carried, but what made Pierson dangerous wasn’t just the shotgun in his hand but the loathsome ideas in his head.