I suspect that we have a coping mechanism wired in where a certain part of our mind tries to refute bad conscience over something we have done that we consider really bad by repeating the deedwhile internally shouting down the horrified parts "see? it's not that bad, life goes on". Repeating the deed, or exceeding. I'd imagine that conditioning (I refuse to call it training) to have been exceptionally effective.

It is estimated that the number of intraspecies killings in humans is about six times higher than the average of all mammals.

The 20th century compared to the Middle Ages, was a peaceful affair. It's estimated that in the good olden times about 12 percent of recorded deaths were inflicted by killing, in the century of two Great Wars, the Holocaust and some minor naughties like the Holodomor, Cambodia, Rwanda etc. just about 1.3 percent.

It seems humans don't have to be taught to suppress a "natural" kill inhibition, but to suppress a natural tendency to kill. You know, I guess there is a reason, God Allmighty had to forbid it explicitly in almost all of his writings from time immemorial. It's always itching the brains of his loverly creatures so much that somehow they can't let it go.

By the way, those Japanese believed wholeheartedly they did it for a greater good and it would strengthen them to inflict severe fear, pain and death on their enemies. They didn't needed to be desensitized.