The emperor's role in Unit 731 is highly debatable; it's not clear what role he played, if any. (Would we hold any particular president of the US responsible for the gain-of-function lunacy that Fauci carried out over the space of three or more presidents?)
At a bare minimum, the sociopaths who ran the experiments should have been hauled off in handcuffs and tried for crimes against humanity; it's a stain on the US that we gave them a pass for their behavior in exchange for the information they gleaned from it.
The historical evidence is available. One article is the Japan Policy Research Institute Paper 92, June 2003, "Emperor Hirohito in 20th Century History: The Debate Rekindles" by SUNY Professor of History Herbert P. Bix.
One might (pathetically) argue that it was better the emperor not be justly tried and convicted of war crimes, but to deny the emperor didn't know or allow such war crimes to occur is completely delusional.