A gun pointed at his gut, the off-duty sheriff's sergeant digs through his pockets with his left hand, fumbling for money, as the three robbers surround him at the Far South Side gas station. The sergeant is surer with his right hand, quickly drawing his own gun and firing point-blank. The gunman, 16, drops at his feet as the two others scatter, according to surveillance video that captured the robbery Monday night at a Citgo station at 103rd Street and Cottage Grove Avenue, across the street from Gately Stadium. The sergeant swings around at the fleeing robbers, then points the...