Joe Zamudio was out buying cigarettes last Saturday when he heard what sounded like fireworks but quickly realized were gunshots. He reached into his coat pocket for the 9-millimeter semiautomatic pistol he carried, clicking the safety off. He heard yelling around him: "Shooter, shooter, get down!" Zamudio saw a young man squirming on the ground and an older man standing above him, waving a gun. Zamudio, 24, had his finger on the trigger and seconds to decide. He lifted his finger from the trigger and ran toward the struggling men. As he grabbed the older man's wrist to wrestle the...