Read Mein Kamph, which was written by Hitler while in prison. In it he laid out the plan that would he would implement during WWII. Nazi Germany blamed the Jewish people and others for their problems, so Hitler used the Aryan race as the poster child as to what the world should look like. So the answer to your question was that they didnt have to justify what they did, they wholeheartedly believed what they were doing was right and good for humanity.