On a cool evening in October, Alain Malcolm, 20, walked into a vacant two-story colonial house in Bristol, Connecticut. Two members of a local internet vigilante group — who regularly try to expose and shame alleged child predators they entice online — were waiting for him. Malcolm was tall and handsome. The oldest son of Jamaican immigrants, he wholly subscribed to the idea of the American dream. In high school, Malcolm was vice president of the Future Business Leaders of America club, assistant captain of the tennis and swim teams and a member of the student council and Model United...