Extermination camp
In 1941, top Nazi official Heinrich Himmler went to see a group of Einsatzgruppen perform a mass shooting.  The group's commanding officer told him that shooting so many people was psychologically harming the soldiers doing the shootings.  Himmler decided the Nazis needed to find another way of killing people.  After the war, the diary of Auschwitz's Commandant, Rudolf Höss, showed that many Einsatzkommandos – the killers – either went mad or killed themselves.  He wrote that they did this because they were psychologically "unable to endure wading through blood any longer."