Top scientists in Japan, Germany and the US all knew that theoretically, the atomic bomb was possible. Both the Germans and Americans worked on it. The Japanese concluded that creation of an atomic bomb would take too long, and instead put their resources into developing biological weapons as their "super weapon." Unit 731 conducted experiments on prisoners, and they practiced on Chinese cities. There are reports of massive epidemics after Japanese planes flew over areas of China. Some reports having over 200,000 Chinese dying of diseases the Japanese were developing.
The Japanese approach was low-tech but highly effective. They selected fleas as the method of carrying diseases, and if memory serves me correctly, planned their first attack in the US with a terra cotta bomb that would have split apart at the maximum dispersal elevation, turning loose over 10,000 fleas infected with smallpox. The target was planned to be San Diego, and the bomb was supposedly already made. The problem was that by the time they were ready to use it, the US had them ringed in so tightly they were unable to break loose a ship with aircraft launch capability and get it to the west coast. They did get some balloons over the US and dropped bombs killing seven people in late 1944, but never dropped a plague bomb. They obviously didn't have a moral problem using it, so I can only assume that by the time they had it perfected, they were unable to get it across the Pacific and deliver it.
The strains of anthrax, cholera and bubonic plague being developed by middle eastern countries and used by Saddam supposedly were originally developed by Japan prior to and during WWII.
When the US dropped the atomic bombs, many said Japan had no idea what the weapon was. This was true of the average Japanese, but the upper echelons of the military and the politicians knew exactly what had happened. The atomic bomb, which they considered theoretical, had been developed.
I hadn't heard the testing story in N. Korea, but Japan certainly had no problem experimenting on their neighbors.
The story of Japan's unit 741 is here.
Actually.the Plague Bomb was a dismal failure,primarily because they could never figure out how to keep the fleas alive at altitude long enough to deliver the bomb, or to keep them alive in an artillery round. Fleas, incidentally, die rather quickly without a host (certainly not long enough to stockpile, so each bomb had to be made within a day or two of it's actual use), never mind the rigors of weaponization. Also, since bubonic plague was endemic in China and surrounding regions, there was no way of accurately counting natural cases and purposely-inflicted cases. You couldn't accurately gauge if it was an effective weapon or not. Certainly not a reliable weapon.
I would tend to doubt the 200,000 Chinese dead figure for the simple reason that in China proper, malnutrition and disease of all kinds were rampant, even before the Japanese got there and perhaps it's only an estimate that an additonal 200,000 Chinese died of disease over and above what would "normally" have died because of the ravages of war (disruption of medical services, sanitation problems, disease caused by unburied corpses, etc all contributing to the total).
I think the only (probably semi-)accurate figure related to Unit 741 was 70,000 (since the facility was supposed to be a lumber mill for security reasons, the Japanese referred to their test subjects as "logs") and that referred to the number of people experimented on (for which there were records made).
Problem with bio bombs is delivery. Fleas are not very hardy, and with the hygenie habits of the US the plauge bomb would not have been that succesfull. But Japan was working on all kinds of nasty things by the wars end.