For years before William Edward Atchison killed two students and himself, he lived a shadow life online—one full of crude alt-right memes and praise for school shooters. In real life, William Edward Atchison—people called him Bill—lived in a little yellow house with his parents, about a mile away from the Giant gas station where he worked and from Aztec High School, where he shot and killed two students and then himself last Thursday. At the murder scene, police found a thumb drive with a note that read, “If things go according to plan, today would be when I die. I...