His hatred for the Jews and his religious beliefs
Also his establishment of concentration camps to inter Jews and other groups he believed to be a threat to Aryan supremacy resulted in the death of more than 6 million people in the Holocaust.
Green, David B. "Why Did Adolf Hitler Hate the Jews?" Haaretz.com. N.p., 11 Apr. 2016. Web. 13 Oct. 2016.
History.com Staff. "Adolf Hitler." History.com. A&E Television Networks, 2009. Web. 13 Oct. 2016.
"Hitler's Religious Beliefs and Fanaticism." Hitler's Religious Beliefs and Fanaticism. N.p., n.d. Web. 13 Oct. 2016.
User, Super. "Was Hitler a Christian?" Was Hitler a Christian? N.p., n.d. Web. 13 Oct. 2016.