I think we can all agree that keeping a cool, calm demeanor in the face of a violent criminal attack is easier said than done. And yet we know it can be done, because it has been done. Remaining calm allows us to deal with such a situation without any wasted motion or wasted effort. It enhances our ability to handle the situation quickly. I used to tell rookie policemen, "You are either part of the solution, or you are part of the problem."
At the OK Corral fight, several eyewitnesses described Wyatt Earp as standing erect, holding his pistol at point shoulder and firing very deliberately. Since there was no autopsy or ballistics exam resulting from that gunfight, we will never know for sure who shot whom. However, I feel certain that Wyatt accounted for Frank McLaury and, along with Virgil, put killing shots into Billy Clanton.
Some years ago, we were on a narcotic raid that went sour. When the shooting started, I noticed my partner line up on a bad guy, then swing his gun far to the right, then come back to the bad guy and fire his shot. When quizzed later, my partner said, "Well, it was dark and I couldn't see my sights. So I swung over and lined them up in the light of that street light, then swung back and made the shot." That would be a good example of being calm and cool.
Some may wonder how trained police officers can encounter a bad guy at arm's length and then miss him with a whole magazine of ammo. In most cases, it's not that this officer can't shoot. We know he can shoot because he qualifies at the police range on a regular basis. I submit that these misses occur because he has failed to stay calm; he has failed to keep his cool.
To a very great extent, calmness comes with confidence. And true confidence comes with training and practice. But mindset is also a critical factor. One must decide he is not going to let criminal trash place him and his family in danger, and he is willing to develop the skills and tactics necessary to see that the criminal doesn't succeed. He can calmly visit violence upon those who would do him immediate harm.