On Thursday, Chris Harper-Mercer, 26, brought guns to Oregon’s Umpqua Community College and opened fire on unsuspecting students and instructors. He killed 10 people, including himself. One day earlier, a South Dakota student shot the principal at Harrisburg High School. The administrator, Kevin Lein, was wounded in the arm. On Sept. 22, a mother shot her children’s father at Central Elementary School in Statesville, North Carolina.