"IG Farben had bought the patent for the pesticide Zyklon B"....
The current main successor companies are AGFA, BASF, Bayer and Sanofi.
IG Farben was not a conglomerate in the American sense. It was a cartel of the German Chemical companies. It was put together in 1925 and broken back up into its component companies after the war.
Farben means dye (from farbe - color), and it was designed to remove competition in the dye and related chemical industries.
It grew out of the World War I collaboration of Hoechst, Cassella, BASF, Bayer, and Agfa. After the first war, the syndicate developed into Farben, with the other companies contributing their assets and taking shares in BASF. While ownership and senior policy making was consolidated at the Frankfurt headquarters (later the HQ for the US occupation forces) but operations remained decentralized. After WWII, the west Germans split out Hoechst and Bayer in 1951, BASF in 1952. (Agfa was initially part of Bayer after the split; then Bayer contributed it to get a half interest in Agfa Gevaert, then bought all of Agfa Gevaert back. And then sold all of Agfa Gevaert in two stages.)
So trying to associate Bayer with a product that neither came from or ended up with them borders on Fake News. You might as well pin it on the French since they have what is left of Cassella and Hoechst.