Your conflating a political argument with a military reality.
The political argument, used to support a variety of leftist causes, is that soldiers aren't natural killers, and as evidence they give the fact that many soldiers in World War 1, World War 2, etc, refused to shoot at other people, and that the military found that soldiers would often intentionally miss even when they were shooting at people, because they didn't want to kill.
The military trains for that, but that has nothing to do with "coolness under fire", which they also train for.
Here's what coolness under fire looks like ... mixed in with a healthy dose of bravado ...
(Note that the camera man was also very cool, or completely clueless ...)
Coolness under fire has nothing to do with killer instinct, it's about retaining your calm during stress, thinking clearly despite circumstances, etc. The military trains for this in a variety of ways .. by subjecting soldiers to a variety of stimuli to desensitize them, by forcing them to think and make decisions while tired, under stress, hungry, while subjecting them to the sounds of explosions, etc.