I’ve never been in a gun fight, but I’ve always imagined the “off switch” is little hard to engage in the heat of the battle, especially if you really believe the person is going to kill you. A sudden, massive overload of adrenaline is probably going to make you do things you wouldn’t otherwise do.
It can be difficult to turn off, and sometimes take a conscious effort. "Ammo discipline" can be a hard concept to teach - particularly the first couple of times on the two-way shooting range. Then you realize if you run through all your ammo on target #1.....you have less for targets #2, #3, and so on.....
You sometimes also have to overcome tunnel vision on the initial target - you become focused on the first target, and miss the others. It's something that requires training for professionals......for the average person it can be akin to on the job training.