By AMELIA NIERENBERG and YAN ZHUANG Ahmed al Ahmed crouched behind a car in a Sydney parking lot Sunday, feet away from one of the two gunmen who had just turned a beachside Hanukkah celebration into a massacre. Then, sirens wailing in the background, al Ahmed jumped into action. Even as the gunman fired a shot in a different direction, al Ahmed ran toward the assailant and pounced on him from behind. The two men tussled for several seconds before al Ahmed wrested a long firearm from the man,...