During the First World War, Bayer turned its attention to the manufacture of chemical weapons including chlorine gas, which was used to horrendous effect in the trenches. It also built up a "School for Chemical Warfare".
After Hitler came to power, Bayer, now called IG Farben, worked in close collaboration with the Nazis, becoming the largest profiteer from the Second World War.
Amongst much else, IG Farben produced all the explosives for the German military and systematically looted the chemical industries of occupied Europe. It's been described as the Nazis' "industrial jackal" following in the wake of Hitler's armies.
During the Second World War, IG Farben used slave labour in many of its factories and mines and by 1944 more than 83,000 forced labourers and death camp inmates had been put to work in the IG Farben camp at Auschwitz in Nazi-occupied Poland.
Not only did thousands of slave labourers die from the conditions in which they worked for IG Farben, those camp inmates who were viewed as too sick or weak to continue to labour in the IG Auschwitz plant were selected for the gas chambers.
When it came to "selection", it was an IG Farben subsidiary, with IG Farben managers on its Management Committee, that manufactured and supplied Zyklon B to the SS. IG Farben also supplied the SS with the Methanol used to burn the corpses. http://humansarefree.com/2016/09/complete-history-of-bayer-one-of.html