It is common for both Hitler and Stalin that both were evil, known as people who may have changed the world most, during the twentieth century in a negative sense. During his reign, Stalin was seen as a "hero" and "savior" of the people, while Hitler was the "Father of the Final Solution"; in other words, both were skillful manipulators, cold and calculating when they came to get what they wanted. They did not respect human life, and they did not have the problem to kill anyone, who would be on their way. Characteristically for Stalin, who was a communist, he was against capitalism, democracy and religion, and he supported the revolution as a way of changing the system and transforming into communism. Hitler, as the creator of Nazism, that is, National Socialism, was an opponent of capitalism, because it is known that he blamed the bankers and capitalists for the economic crisis in Germany, as well as the "decadent democracy". He was not a communist because his conception was based on raising "higher race" as the descendants of the Aryan race, and a natural choice that implies the survival of the most capable, in other words, the survival of "higher race" and not workers and peasants, ordinary people as in communism. Also, although he was not benevolent to the Catholic Church, he was generally not against religion, because, according to unconfirmed sources, he practiced some forms of pagan rituals.
The answer is: against capitalism.