What happens in a state where ordinary people are able to carry guns? Mayhem? No. What you tend to see are under-reported incidents like this.
Charlie Merrell was in a checkout lane at an Indianapolis IGA Supermarket, when a masked man jumped over a counter and pointed a gun at an employee. Merrell drew his own handgun (for which he had a permit), ordered the robber to drop his weapon, and held him until police arrived.
If the robber had shot a bunch of people, this would be all over the news. (In this case, the robber's gun wasn't loaded.) But an incident like this gets little coverage; this imbalance is part of why so many otherwise rational people have such fear of citizens carrying guns.