It's important to note here (as [@Shoryu Magami] touched on) that military training has an impact on this. Modern military training specifically is designed specifically to make you shoot first and think later. Nowadays, if you're in the Army with a gun and you come across and enemy, you'll fire on instinct. In WWII, however, MOST of the soldiers wouldn't even pull the trigger. This goes all the way back to the invention of the musket, where the accuracy of fire reduced rapidly in actual combat due to the simple fact that these people were staring other humans beings in the eye and consciously or unconsciously threw off their own shot. There are plenty of stories of opposing forces meeting during these time periods and simply walking past each other. One case from one of the world wars involved small groups of British and German forces meeting along a road. Instead of opening fire, they laid down their weapons and started [i]throwing rocks[/i]. Lindybeige (my personal favorite military/history nerd channel) actually has a video on this specifically and touches on how it can affect reactions to killing (briefly, if I remember right, but he still touches on it).

[youtube]https://m.youtube.com/watch?v=zViyZGmBhvs[/youtube]