Tapping on his cellphone with a sense of purpose, Kevin Mathewson, a former wedding photographer and onetime city alderman in Kenosha, Wis., did not slow down to fix his typos as he dashed off an online appeal to his neighbors. It was time, he wrote on Facebook in late August, to “take up arms to defend out City tonight from the evil thugs.” One day earlier, hundreds of residents had poured onto the streets of Kenosha to protest the police shooting of 29-year-old Jacob Blake. Disturbed by the sight of buildings in flames when he drove downtown, Mr. Mathewson decided...