What is the worst job in human history?

Scientists belonging to Unit 731.

The unit's official name is Kantōgun Bōeki Kyūsuibu Honbu, Department of Outbreak Prevention and Water Purification. However, this unit actually conducts experiments on humans.

The unit commanded by General Surgery Shiro Ishii is known to have carried out various gruesome activities as follows:

• Surgical life (vivisection).  Thousands of men, women and children, some of whom were prisoners of war, have been victims of surgery where most of them were operated on without anesthesia and eventually died. Initially they were injected with certain diseases, then their bodies were dissected and their organs removed to determine the effect of the disease on human organs.
• Research on biological weapons and venereal disease in which prisoners of war are injected with syphilis and gonorrhea.

The building of a biological weapons test site in Harbin.

• Research on the effects of organ freezing. The legs or other body organs are frozen and then hot water is poured on them and beaten with sticks.
• Trials of weapons such as grenades and fire extinguishers against living beings.