Saying keep their colonies as holocaust is overkill. Western countries also had their colonies.
Western powers weren’t committing atrocities like the Rape of Nanking during the 30s and 40s. Japanese troops killed surrendered soldiers, raped and murdered civilians not as a rare or exceptional event, but as a standard act in every country they invaded during the 30s and 40s.
And from the Allied perspective, after the Bataan death march, the massacre of surrendered US troops and civilians in Wake, and the hundreds of similar massacres of surrendered allied troops, and rape and murder of nurses and civilians, across the Pacific, no one was going to allow the Japanese leadership or military off the hook.