In World War Two, it is a fact that only 15-20 percent of the soldiers fired at the enemy.

One of the originals is Men Against Fire by World War I Officer S. L. A Marshall.

You find this claim all over the place; the problem with it is that comrade "S.L.A.M" is not "one of the originals", he is the sole and only source for the claim - and he made it up. A cursory Wiki search shows:

[So-and-so demonstrated] that Marshall had not actually conducted the research upon which he based his ratio-of-fire theory. "The 'systematic collection of data' appears to have been an invention."

My emphasis.

The best evidence we have is that killing is in fact difficult for most people, most of the time, even in war.

Ok. So on the one hand we've got a single book, later shown to have been an invention, but taken up by a huge number of people so it looks like a consensus, in the best Dark-Arts, "you have to be smart to know this", counterintuitive-Deep-Wisdom style. And on the other hand we have a huge amount of dead people, mysteriously killed by bullets that, somehow, got fired in spite of the noted reluctance of men to do so. I propose that your accolade of "best evidence" is a bit misplaced.

This is an excellent example of the need to apply some skepticism to a counter-intuitive but neat-seeming claim, whose possession will put you inside the tribe of people who Know Neat And Counterintuitive Stuff. Sometimes the simple answer really is the right one; this is one of those times.