The statistics about US GIs not trying to kill enemy soldiers during World War II are based on conclusions by historian SLA Marshall who died in 1977, and they have been contoversial since they were published. Marshall said 25% of troops didn't fire their weapons, and as the statistic passes from hand to hand it gets to be less and less apparently. Since a soldier has to expose themself to enemy fire to shoot, one who hasn't fired his weapon has more likely done so out of self presevervation rather than any reluctance to kill. You can't argue that the US Army and USMC didn't leave piles of dead German and Japanese soldiers behind them where ever they went during the war. By the Vietnam War small unit training and the weapons had improved in part due to studies like Marshall's, helping the troops to get their volume of fire up.

And there has been substantial gun reform over the last twenty years as more and more states made it easier to legally carry concealed handguns. 22 states now have unrestricted, permit free concealed carry.