And maybe I should clarify a bit. Every situation is different. You always have to use your best judgment and try to read the situation. This is difficult without training for that specifically. Law enforcement officers spend extensive amounts of time training on that. In the SF CQB course, you spend almost three months learning to discriminate targets and only hit the right ones, the bad guys. One round on an innocent and you are dropped from the course. It doesn't matter that the innocent was behind the terrorist target. When people without that sort of training get into gunfights, it looks more like a gang war with bullets going everywhere.

As I said, every situation is different. If someone were breaking into my house, in order to protect my family, the first thing I will do is work the action on my pump 12 gauge, then yell out that I have already called the police, then I will call the police. I would rather not shoot in my house unless absolutely necessary. Most criminals are intent on surviving and not getting caught and will immediately vacate the premises.

If I found myself in the area while two armed people were threatening each other with firearms, I will seek cover and get as many other people to cover as possible. Then I will call the police. A gun coming out at this point will only make matters worse.

If someone is intent on killing, such as a workplace shooter or school shooter, there is no deterrence. They are usually intent on dying as well. You must eliminate the threat as quickly as possible in any way possible.

It is not natural for one human to kill another. There are strong psychological barriers to this. These barriers must be overcome, even in soldiers (I can quote a huge number of studies to support this) and they must be trained to overcome it. There is a process for doing this (see Bandura on "The Mechanisms of Moral Disengagement"). If someone is already carrying a gun and threatening (or trying) to kill, they have likely already gone through the process and are very willing to kill.

My usual choice is to be as non-threatening as possible until I have to take action, then I take the action necessary. This does not necessarily involve a gun, in fact even if I have one I will avoid pulling it if possible. In situations where you are likely to need a gun, once you pull it out you should be ready to use it.

Sorry for the psychobabble, but I am pursuing a PhD in Psychology. I know; it is an unusual combination of a former SF soldier and psychologist.