He was the Sergeant-At-Arms of Canada's House of Commons in Ottawa, a largely ceremonial position, when a jihadi burst into the building bearing plenty of weapons and ammo. But because Vickers happened to have a pistol in his desk drawer and the military training that meant he could use it effectively, he was able to pick off the gunman before he could do any damage. (Any damage inside the building, that is; the jihadi had already murdered the Canadian soldier standing on guard at the Canadian War Memorial.)
Canada has "gun control" up the wazoo. And a fat lot of good it would have done us that day if Vickers hadn't had his gun at hand and the wherewithal to use it. Because he had both, he was able to prevent a bloodbath whose casualties would have likely included many MPs, and perhaps even the prime minister himself.