This is actually a factor I'm less convinced is relevant. A relatively small proportion of casualties are due to small arms fire. Artillery and mortars are the main killers on the battlefield. I doubt willingness to commit criminal homicide is correlated with willingness to lob HE shells to enemy positions kilometers away.
I'm not sure if this inspired your post, but On Killing by Dave Grossman was widely influential but not highly regarded among military people. The combat veterans I've talked to reported little to no hesitation among them or the men they led to use lethal force against the enemy. They reported not only an absence of hesitation, but an eagerness to claim credit for dispatching the enemy (to the point of getting into arguments over who killed which enemy).
Indeed! Modern artillery and mortars are indeed widely lethal. As are air strikes, missiles, tanks, IFVs, etc.
Boots on the ground are still what hold it though, and that still requires the willingness to do what I was describing, even if they usually aren’t called to do it because other, more effective means do it first.
Being able to handle being in reach of artillery and mortars, also required, once you know the above is true.
Additionally, someone willing to bayonet a stranger is going to really have no issues pulling a lanyard on a howitzer.
I'm not sure if this inspired your post, but On Killing by Dave Grossman was widely influential but not highly regarded among military people. The combat veterans I've talked to reported little to no hesitation among them or the men they led to use lethal force against the enemy. They reported not only an absence of hesitation, but an eagerness to claim credit for dispatching the enemy (to the point of getting into arguments over who killed which enemy).