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.
I have never bought that Marshall stuff, it didn't make sense, and just wasn't credible, nor did it fit countless battles or human nature.