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
There were lots of other Japanese war criminals.
If they had been pursued down to the concentration camp guard for two months level that the Nazis have, we’re talking perhaps hundreds of thousands of them.
BTW, given the assumption, probably correct, that USSR was indeed working on biological warfare, the amnesty given Japanese scientists is not entirely unjustificable. Damaging our chance of surviving a future war in order to punish the perps from a past war isn’t a great idea.
There is also a great deal of debate about the extent to which the Unit 731 guys actually came up with any valuable knowledge. It is likely that many of their “experiments” were essentially torture sessions without much real scientific value.