One Sunday afternoon last month, a hundred boisterous patrons crowded into Mad Dog in the Fog, a British sports bar here, to watch a live broadcast. Half the flat-screen TVs were tuned to a blood-filled match between two Korean competitors, “MC” and “Puma.” The crowd erupted in chants of “M-C! M-C!” when the favorite started a comeback. The pub is known for showing European soccer and other sports, but Puma and MC aren’t athletes. They are 20-year-old professional videogame players who were leading computerized armies of humans and aliens in a science-fiction war game called “Starcraft II” from a Los Angeles convention center. The Koreans were fighting over a tournament prize of $50,000.