Juan Carlos Nazario was sitting on a lakeside bench waiting to play soccer when he heard the staccato popping of gunshots outside Louie’s On the Lake, a popular waterfront grill and pub. He ran to his car to get his gun and moved toward the sounds. Bryan Whittle was driving with his wife, heading off for a Memorial Day weekend getaway, when he saw a commotion outside Louie’s. He thought someone might be drowning, so instead of turning his truck onto the highway, he barreled into the parking lot to offer help. As he jumped out, what he learned stunned...