He conducted his experiments on Auschwitz prisoners during the holocaust. He was a doctor in Auschwitz who used the prisoners to conduct highly unethical experiments such as taking people's organs without anesthetics for examination, or freezing people to death in order to examine effects of hypothermia.
He believed that he was not evil but that it was a great opportunity to expand science. Since he didn't consider Jews to be people, they were good for his experiments since he could do anything he wanted while rationalizing that it was ultimately worth it since he would get new information and develop medicine so he believed that the ends justify the means.