The human brain doesn't always function correctly when adrenaline gets pumping.
It depend on the person. It depend on how you act in your life.
People who live their life as if they were in some kind of utopia with no crime are the one who cant act when faced with life threatening situation. On the other hand many people can still act in these situation, even people who never fough in war, they simply are always aware of their surounding "just in case something happen".
You can see the difference when someone save other from a robber or killer but at the exact same time you see many people unable to even move their ass.
Many people who fough in war and killed human actualy tell you that nothing changed. People believe that there is a massive change because the one you ear the most are people with PTSD, but most soldier are the same at the start and the end. They dont go trough some change that make them better at supporting pressure and suddent event.
The main thing that make them able to act proprely is their training.
That's why all good special force spend most of their time training for all possible senario.