The extraordinary feats of ordinary men are the things of heroes. This was on full display this week in Las Vegas. Merconie Clark, a Southern California truck driver, had gone into the Smith’s at Maryland Parkway and East Silverado Ranch Boulevard on Tuesday late morning to pick up a few things before hauling a load back to the Los Angeles area. When he heard gunshots, he headed to the pharmacy section to seek cover. Then a store worker yelled, “There’s the shooter!” The 49-year-old Mr. Clark didn’t hesitate. “I don’t know what happened,” he said later. “I just walked over towards him, and he pulls out his gun, and we start wrestling for the gun.” During the struggle, Mr. Clark called for help. Darius Alston, 28, works at a nearby barbershop. But on that day he went to Smith’s to help the mother of his children grocery shop for their two young toddlers. When Mr. Clark found himself fighting for his life, Mr. Alston jumped in. “That’s when Darius hit” the gunman, Mr.…