I think it’s important to differentiate between draftees and volunteers. WWI and WWII soldiers were mostly draftees with minimal training. It was not like what we have today with an all-volunteer army. There were high levels of panic in new troops. It is likely that element of pacifism or desire against killing was a portion of the result, but it doesn’t make sense to chalk most of it up to that.
I don’t have access to it now, but I’ve done research on the difference between green troops and veteran troops in WWII, and the numbers are night and day. Green troops fit the profile of people who die quickly and often don’t use their guns at all. Experienced troops do not fall into that category.
I think it’s also important to emphasize the fact that there are reasons to shoot other than to hit someone. In modern warfare with air support and artillery, it makes sense to do just enough shooting to keep the enemy contained, then let the big weapons take care of the problem. Even if you’re committed to an infantry action, you do a lot of shooting just to make sure the enemy keeps his head down, doesn’t charge your position, etc.
What the statistics tell me is not that people are averse to killing, but that they are averse to being killed. They’d rather stay behind cover than shoot. And they’d rather skimp on aiming than shoot well.