ON OCTOBER 1, 2015, in a scene that has become depressingly familiar, 26-year-old Christopher Harper-Mercer walked into Umpqua Community College in Roseburg, Oregon, and opened fire, killing nine people and injuring nine more. Like George Sodini, who killed three women and himself in an L.A. Fitness Center in Pennsylvania; like Seung-Hui Cho, who killed 32 people and injured 17 others at Virginia Tech; and like Elliot Rodger, who killed 6 people and injured 14 others in Isla Vista, California; a large part of Harper-Mercer’s motivation was his toxic anger at women because of his lack of success with them. Harper-Mercer...