That is not quite accurate. Victor Davis Hanson deals with this very question at length in several of his books; the "Western (American) Way of War" where he repeatedly remarks on the unique phenomenon in history, where brutality was not only needed, but essential, in order to win against a barbaric, truly evil and merciless culture, and the victors, immediately afterwards, went right back to their occupations of farmer, doctor, teacher, father, without any overt evidence of the brutality they practiced just months earlier.
A unique phenomenon in all of history.
Except for those self-centered fools, traitorous fools, whose grasp of history simply emcompasses their own pathetic existence...
Interestingly, so did the losers.
Very few Japanese war criminals were prosecuted and punished. Yet it appears the vast majority of these "evil men" went right back to living normal, law-abiding lives, with Japan in fact having much lower crime rates than the victors. This includes, of course, almost all the survivors of the perpetrators of the Nanking massacre, the Bataan march and all the other Japanese atrocities.
The same appears to largely be true of the European "war criminals," mostly concentration camp guards, still exposed occasionally in USA. Most of them lived perfectly normal, law-abiding, even admirable lives for three or four decades after their crimes during the war.
We generally think of "war criminals" as being different from the rest of us, with the implication that there's something wrong with them and that they are by nature criminals and brutes.
The evidence is quite otherwise. It appears many, if not most, humans are capable of this type of behavior under the right circumstances, and equally capable of returning to normal lives when the conditions change.