One thing I learned recently that's probably contributing to the problem is a little complex.
See, back during the late WWII and early Korea period there was this guy called S A Marshall. He had this theory that people have a deep-rooted aversion to killing, and this contributed to a massive reluctance of newly-trained troops to actually engage in combat. He supported this with a lot of troop interviews both group and solo, and published his findings, recommending changes in training to break down this aversion.
Much later, a Colonel named Dave Grossman dug up these old findings and built a whole range of training material designed to do just that. Besides being the basis for a lot of the "murder simulator" rhetoric built around violent games, it was also adopted not only by the military but by a lot of police forces on the grounds that if a cop is in a position where he must resort to force, hesitation risks the lives of not just the cop but innocent bystanders as well. This is because that is the engagement threshold that cops are supposed to have programmed in during training.
The problem is that Marshall's work was bunk, but the derived material works. Commanders don't report any ingrained reluctance to fire - quite the opposite. The biggest problem with raw troops was their tendency to start blasting away at every shadow out of fear. Investigation proved that most of the interviews he recorded either didn't happen, or were nothing more than a five-minute session that he "extrapolated" from. Moreover, even in his actual interviews he went out of his way to explain away valid reasons not to shoot -not actually being in contact during that engagement, being ordered to hold fire for tactical reasons, uncertainty of target identity, things like that. Most people going over his work have concluded that his much touted "aversion to killing" either does not exist, or is far weaker than he claimed.
However, Grossman's training programs based on that "research" do, in fact, successfully make trainees more comfortable with the notion of killing. Since the aversion was a myth, that has had the result of seriously eroding the engagement threshold for lethal force.