I think it's situational. Activities that become normalized during wartime often aren't acceptable during peacetime. Even then there can be differences. WW2 vets, although many were traumatized etc, in general came away from that war in a different condition and frame of mind when compared to Vietnam vets, where there wasn't widespread acceptance of and approval for their actions.
We might be coming to the same conclusions from different perspectives. Imo, a propensity to violence is part of the human motivational repertoire. I believe that our society's attempt to deny that fact is wrongheaded, just as it's wrongheaded to try to ignore the human sexual impulse. I'm not saying that it should be ok for people to kill in order to work off some steam; but that cathartic activities should be recognized to have utility in channelling that violent impulse away from being acted out in daily life. I think the Japanese have a better understanding of this than we do.
Getting back to your point: yes, it's possible for otherwise mentally 'normal' people to kill, but depending on the circumstances, certain types of killing will more often be within the province of the mentally or emotionally unbalanced.