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.
There were plans to unleash horrific plagues onto the US mainland by balloon which as a matter of fact the Japs did manage to accomplish to a limited degree with balloons having landed in Washington State during the war.
The medical facilities of Unit 731 as it was called was every bit as diabolical, horrific as the Nazi medical experiments.
Much to its discredit the US granted immunity to those of Unit 731 in exchange for their research in an effort to best the Soviets in this unexplored arena of bacteriological weaponry.
For more information on Unit 731
Wikipedia:Unit 731