Chilling surveillance footage captured the “Wild West” moment an NYPD cop shot a gun-toting maniac who allegedly fired at officers after threatening to shoot up Mount Sinai Hospital. The tense clip showed the gunman – 20-year-old Elijah Brown – holding his weapon in his hand as he casually walked on Madison Avenue between East 95th and 96th streets late Thursday after telling a bodega worker he planned to riddle the hospital with bullets.  As cops approached Brown, he suddenly whipped around