Something that gets left out of this is the flinch factor, fear, I read once that in a firefight, the advantage often goes to the side that first gains the upper hand, because it is difficult for men to force themselves to expose their face to the incoming bullets and violence to return aimed fire, and take control of the situation, when that control is already the other side’s.
The advantage in quality troops or elite troops, is that individually they are more able to force themselves to face the enemies dominance and incoming bullets, to assert the return fire necessary to take away that dominance, and to force the switch-over to get the enemy cowering and losing control over the situation.
I can see how in some situations, troops just get overwhelmed at the beginning with the result that many just hunker down, and have a low rate of return fire, (at least effective return fire).
Having lived that situation, I would say that what you read is correct. But that is not driven by some psychological barrier to killing enemy soldiers, it's driven by fear and also by uncertainty about what to do. Training and leaders are the remedy. When soldiers understand what you want them to do, they do it. Returning fire immediately, building up a volume of fire quickly, and then reacting with fire and maneuver are characteristic of trained and aggressive units. Such units usually take far fewer casualties than their opponents.
You need to do a little more research. Try this:
He had been in that heightened state of awareness some soldiers enter into when engaged in close combat. Some say that when they’re in this state, all their senses are focused on killing the enemy. Their senses are magnified to the point their vision can detect and identify the smallest detail; they can hear a leaf fall out of a tree at a hundred meters and identify the type tree it fell from; they can smell the enemy’s breath and identify what they ate for their last meal, and their physical strength is magnified three fold. Soldiers have been shot through the heart while in this heightened state and have continued to fire, empty a magazine, change magazines, empty it again and throw a grenade before their body falls down and dies. Some call this heightened state an “adrenalin high,” and some simply call the state “Berserk.” What it really is I don’t know, but I do know it exists.
http://www.projectdelta.net/wrong_valley.htm