Full disclosure: I am a friend of marsmrsmars.
It comes off as weirdly paternalistic whenever you say that people who want to play on this MUSH "need to have a certain level of tolerance with uncomfortable history" in order to play here. First things first: the game does not bill itself as "dark" or "mature," the staff already went through the trouble of removing Adolf Hitler from leadership in Germany and outright banned ex-Nazi characters, so I don't see why there is any hand-wringing about ex-IJA characters.
Second of all: people in this thread are weirdly veering toward outright revisionism whenever they question whether or not Hirohito had any involvement in his country's crimes. His own family participated in them, and he was certainly aware of them. This is elementary school history. Any denial of that shows a complete lack of understanding, and you should not be speaking on a topic that you do not understand.
EDIT: It also comes off as oddly voyeuristic whenever people suggest that they're attracted to exploring these topics for whatever reason. As a roleplayer, I think you should consider that the themes you are interested in exploring are in a real context in this instance, and directly related to terrible suffering that is in living memory of some people alive today. If you WANT to play as a war criminal that participated in the massacre of 25 million Chinese people, I think you need to reflect on why that is.