For A.M. Hickman, it was reading Ted Kaczynski’s manifesto online when he was 13. Soon he was posting to survivalist message boards. By age 21, he was on Reddit, moderating one of the internet’s largest forums advocating for “liberation" from the government, capitalism and other systems members saw as oppressive. While the two young men fell into very different internet rabbit holes—one with alt-right leanings and the other on the far left—they were both drawn by a desire to find belonging and a yearning, they say, for the hero’s journey. For both, the result was depression and loneliness. They each...