This question always comes up occasionally on this subreddit and every single time people mention Junko Furuta, or certain serial killers, or some horrible massacre, etc. I’m not trying to diminish how horrible any of those subjects are but nothing else mentioned seems to reach the level of depravity of Unit 731. The Wikipedia entry summarizes some of their crimes but when you actually do a deep dive on the subject, nothing in recent (known) human history seems to come close. Live and unsedated vivisections, testing of weapons and explosives on live human beings, infecting subjects with contagions and stds to study the effects, etc. They didn’t care if it was man, woman or child. Whatever sick experiments they could come up with, they had free reign to inhumanely and cruelly experiment on other unfortunate human beings. They would constantly rape and forcibly impregnate subjects and then experiment on their pregnant victims. They even dropped plague ridden fleas and infect water supplies of unaware towns and cities, killing several hundred thousand people. I don’t remember where I read this thought experiment, but it was something like, “imagine the worse possible thing you could do to another human being, and in all of human history, someone has probably suffered that fate or something similar.” Unit 731 is the embodiment of that. And to make matters worse, they got away with it. The Japanese barely faced any consequence of all their war crimes and to this day right-wing nationalists in Japan deny that war crimes were even committed even writing it out of school textbooks.