The ultimate price would have been for the Allies to stand firm on the July 1945 Potsdam Declaration and, following Japan’s surrender, to tried, convict, and sentence the Japanese emperor for such war crimes.
It was General Douglas MacArthur who protected the Japanese emperor. He should have been court-martialed for that.
Executing the Emperor would have led to the immediate shattering of the surrender and peace process. The Japanese would never have accepted such an action and it would have led to years more conflict and guerrilla warfare that would have made Vietnam look like a tea party. The United States did the right thing and the results are obvious today.
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.