People nickname him as Angel of Death because he research, experiment on human subjects that mostly lead to death and someone recalled one occasion where Mengele personally killed fourteen twins in one night through a chloroform injection to the heart in a night. 
He is German Schutzstaffel (SS) officer and physician during World War II.
He's give them false sense of  security like being very friendly to children  and often give candy to the little ones until the experiments.