re: Whats the most horrific torture in human history?

Prisoners of war were subjected to vivisection without anesthesia.[15] Vivisections were performed on prisoners after infecting them with various diseases. Scientists performed invasive surgery on prisoners, removing organs to study the effects of disease on the human body. These were conducted while the patients were alive because it was feared that the decomposition process would affect the results.[16] The infected and vivisected prisoners included men, women, children, and infants.[17]
Prisoners had limbs amputated in order to study blood loss. Those limbs that were removed were sometimes re-attached to the opposite sides of the body. Some prisoners' limbs were frozen and amputated, while others had limbs frozen then thawed to study the effects of the resultant untreated gangrene and rotting.
Some prisoners had their stomachs surgically removed and the esophagus reattached to the intestines. Parts of the brain, lungs, liver, etc. were removed from some prisoners.[15][18]

Unit 731, wiki it.