In my eight years in DC there was one occasion where a clearly aggressive, violent and deranged person might have gotten shot if people had carry permits. The Metro was slow due to an outage and we were on the platform for about an hour. During that time period, this very muscular guy in his forties, homeless looking, kept following this group of college girls around loudly saying he wanted to show them a good time and they could either let him fuck them or he would rape them, right here right now. Of course somebody called the cops, but it took 20 minutes for them to show. I intervened when it was clear the girls were scared and he was physically herding them into a corner, and he completely lost it. He was screaming at me to mind my own business or he'd kill me and he got almost within arm's reach before I calmed him down to a low buzz by the time the cops arrived but it was scary and unpleasant. And even though there were dozens of people on the platform, I got no help whatsoever. Even the guy who had been with the girls peeled off and let them fend for themselves. The girls were very grateful and I told them they needed better friends in front of the guy.
If a marine had choked him to death, I would not have objected. Although, candidly, I was really most concerned about being bitten or scratched. I doubted he had a weapon.