Yeah I know Japan has its own problems but why such little violence?
I hate to sound cold, but they took every kid they could find with aggression and violence, and threw them into war with an order to fight until they ran out of ammo and then make a kamikaze suicide run, in a war they lost pretty decisively.
It is possible mankind is only kept civilized by existing in groups who expel the unmanageable into the cold, and cull them from the population.
We have not had such a selective pressure for a long time, so we may be culturing violent mutants in ever increasing numbers, and allowing them to spread genes and multiply.