The 20% are in it because they love it. They are perfectly willing to die in order to be exactly where they are. The idea of being in a war, bloodied and possible dying is the price they pay for the rush they get. The adrenaline helps their instincts, gets them pumped up and they can stay awake for days, do their jobs and do it well.
I know this effect is real but I also know, as you said, that not all people respond the same way.
If you will forgive a personal account here is another reaction. Once I was confronted with a situation in which there was a very good chance I would not survive. The details are not important except to point out that it did not catch me unexpectly. I had a few hours to think about it. When the event did happen, I was surprised at my physical and mental reactions.
First of all my mind became crystal clear. I had a feeling of invincibility. I am sure the feeling was to an extent false, yet I also think I really would have been able to handle any situation better than at any other time in my life. I cannot stress enough how confident I felt. I absolutely knew I could deal with anything. Fortunately I did not have to test my new found powers.
I have never read anywhere of that effect yet am reasonably certain most others would react the same way in similar circumstances.
I would be interested if any others have experienced the same.