Killing is traumatic even for people in the USA.
The difference is that in America most people have the option to carry guns for protection and many of us make peace with the fact that we may have to use them to protect ourselves in life or death scenarios. Most Americans who carry firearms for protection have already rationalized and come to terms with it to some degree. It doesn't completely negate the psychological affects, but it allows us better ways to cope compared to a Japanese person who has likely grown up in a non-violent and largely pacifist society. And even then, it kinda varies from person to person because everyone reacts differently to violence. You don't really know until it happens.
America is the melting pot of the world, but that doesn't come for free. Tribalism between ethnic groups are common which causes a lot of conflict that can escalate to violence. Conflict is a significant part of America's history, and the American people tend to be more resilient to it as a result.
