There is a commonly quoted statistic, albeit controversial and debatable, that only 15% of American riflemen fired their weapons in WWII, and only 2% shot to kill

I think its important to underline just how "debateable" these stats are. They are, according to most sources, nothing less than a fantasy.

From Roger J. Spiller, "S.L.A. Marshall and the Ratio of Fire", The RUSI Journal, Winter 1988, pages 63-71:

"Why the subject of fire ratios under combat conditions has not been long and searchingly explored, I don't know," Marshall wrote. "I suspect that it is because in earlier wars there had never existed the opportunity for systematic collection of data."...

By the most generous calculation, Marshall would have finished "approximately" 400 interviews sometime in October or November 1946, or at about the time he was writing Men Against Fire.

This calculation assumes, however, that of all the questions Marshall might ask the soldiers of a rifle company during his interviews, he would unfailingly want to know who had fired his weapon and who had not. Such a question, posed interview after interview, would have signalled that Marshall was on a particular line of inquiry, and that regardless of the other information Marshall might discover, he was devoted to investigating this facet of combat performance. John Westover, usually in attendance during Marshall's sessions with the troops, does not recall Marshall's ever asking this question. Nor does Westover recall Marshall ever talking about ratios of weapons usage in their many private conversations. Marshall's own personal correspondence leaves no hint that he was ever collecting statistics. His surviving field notebooks show no signs of statistical compilations that would have been necessary to deduce a ratio as precise as Marshall reported later in Men Against Fire.

The "systematic collection of data" that made Marshall's ratio of fire so authoritative appears to have been an invention.

While John Whiteclay Chambers II, "S. L. A. Marshall’s Men Against Fire: New Evidence Regarding Fire Ratios," Parameters, Autumn 2003, pages 114-121 states

without further corroboration, the source of Marshall’s contentions about shockingly low fire ratios at least in some US Army divisions in World War II appears to have been based at best on chance rather than scientific sampling, and at worst on sheer speculation.

It seems most probable that Marshall, writing as a journalist rather than as a historian, exaggerated the problem and arbitrarily decided on the one-quarter figure because he believed that he needed a dramatic statistic to give added weight to his argument. The controversial figure was probably a guess.

In other words, Marshall simply invented some stats based on some fantasy he had and the establishment swallowed them whole.

Dave Grossman took these stats even further and in his book On Killing adds even more dubious anecdotes to the bonfire of historical rigour. This includes the mention of an American Civil War musket that was recovered from the battlefield and loaded multiple times - indicating that the person carrying the musket had a mental block against firing so just keep putting new loads down the muzzle. Only thing is, there is no evidence that this musket actually existed at all.

Finally, its very easy to say that because the allies never perpetrated atrocities on anything like the scale that the Nazis did, that their attitude to killing was "opposite". There was in fact a good deal of rather nasty behaviour, including the practice of 'skull stewing' by US forces in the pacific.

Even today, we see instances of the soldiers of democracies going a bit heart of darkness, and collecting ears and fingers from their 'kills'

German history is not my specialty, but the ideological nature of Nazi war offers some explanation to your question. Nazi Germany's genocidal politics and its obsession with rooting out an internal enemy were part and parcel of its war-making methods.

National Socialist war is what some historians call radical, as opposed to the conventional war practiced by the Allies, or even by the German Empire in World War I. Ideology motivated German soldiers with a focus on "collective will" and performance. German soldiers clung to a strong group attachment. Every man was expected to achieve to the best of his ability and to know his role perfectly, drawing from tactics developed in the Great War. This attachment to "will" made war an almost spiritual act, and clearly as a cohesive bond for the army it worked, because unlike in 1918, in 1945 the German army pointlessly fought to the last bullet.

This performance-based system valued unrestrained efficiency, the concentration of overwhelming force that has been lumped together as "Blitzkreig" tactics. This meant no effort spared, and entailed use of force on civilian populations. "What we find is a conduct of war that encouraged the use of force, unconstrained by rules of war. The normativity of excess, captured in the holy grail of performance-driven command tactics, holds the key to the German conduct of war."

The need to drive this war machine entailed occupation and exploitation of the occupied zone through terror. "The German conduct of war built on achieving total submission and absolute superiority," hence the brutality of state security like the SS. The intended effect was to create a sense of German superiority that both cowed occupied populations and made confident German troops abroad.

This potent mix of terror and German chauvinism led to catastrophe in Eastern Europe. Unlike in France or Scandinavia, the Nazis intended to destroy Eastern European states. Therefore there was no reason for restraint. An ideology of unrestrained violence, German superiority, Slavic inferiority, and the value of total force meant that extreme violence was standard orders for German soldiers in the east.

Konrad Jaurash & Michael Geyer, Shattered Past: Recovering German Histories