"Desensitization" is the term. US military surveys done during WWII found that the majority of troops refused to shoot at the enemy (or at least claimed they wouldn't shoot). They'd empty the magazine. They'd march up the beach to almost certain death.....but were (at least saying they were not) shooting at the enemy, for risk of taking a life.

At the time, marksmanship training was still done with bullseye targets. Moving to silhouette targets (part of the ensuing 'reforms') had a statistically measurable effect on performance. As training methods improved through Korea and Vietnam, the survey results improved. It's been a while, but I think it took until Korea for the surveys to report over 50% shooting at the enemy. By Vietnam it was ovr 70%, if I recall correctly

Today, it's opposing force training with laser weapons. And then there are the video games.....both recreational and training. Add to that a self-selecting (volunteer) force, and you're probably looking at close to 100% willing to use lethal force.