Found an interesting article on the su8bject.

A study done by U.S. Army Brigadier General S.L.A. Marshall, after WWII, determined that only 15-20% of our troops fired their weapons with the intent to kill or wound the enemy.

Other studies have found that this has pretty much always been true. In the Civil War, captured muskets were fully loaded in 90% of the cases and of that 90%, many were found to be loaded more than once. This would indicate that the engagement of loading one's weapon was a great excuse for not firing it.

After studying the results of Marshall's research the Army instituted training procedures that increased the firing ratio to 55% in Korea and 95% by Vietnam. This dramatic increase is blamed for the high incidence of PTSD after Vietnam.

While all of the ratios are somewhat questionable to me I have found from personal experience that unwillingness to fight physically and aggressively is a common trait in humans. If you think back over your own experiences you might come to the same conclusion.

I also believe that the incidence of PTSD is better explained by returning home to find the people you fought for, believe your efforts were unjustified and unethical than it is by the horror of taking a human life. While it is no doubt horrible, I believe seeing it done by others at close quarters would be equally stressful.