To preface this I'm a Political Science major and a history Minor. Last semester I took Russian Government and Politics and WWII in Europe so the semester covered both Hitler and the politics of the reign of Stalin.

When comparing the two leaders it's very important to look at their goals and what they were going for. Stalin was heavily focused on personal power and attempting to create a USSR which was under his absolute control. This made him domestically focused. While he was directly responsible for the purges which killed a massive amount of people most of the deaths during Stalin's rule came from his policy of collectivization. This policy reformatted the farms of Russia and forced the people to give large quotas of their harvest to the government. Whenever people talk about Stalin causing mass death of his people they're usually referring to this policy because of the famine it caused. The goal of collectivization was a communist solution to an economic issue that obviously failed. The famine it caused was not intended by any means.

Hitler on the other hand was bent on world conquest. His goal was to defeat Europe and eventually fight the United States, win and achieve global dominance. Hitler was also intent on killing every single Jew in the world. In fact he was absolutely obsessed with this to the point of dedicating resources near the end of the war to killing as many Jews as possible. Hitler believed a global Jewry existed which would bring down Germany if they were allowed to exist even in small numbers. The Holocaust was a campaign of pure murder. Stalin's purges sought to eliminate political enemies and threats to his rule. While the majority where innocent victims the rationale is at least somewhat sane as Stalin feared anyone who was not completely on his side. For Hitler however there was less of a political motivation and more of a racial one. This is generally considered the more evil of the two.

Another major point you seem to be missing is that Stalin only killed more people because he lasted longer. The major of the deaths he caused being from the famine that ensued from his terrible economic policies and then the poor military decisions he made facing Germany. These are more or less indirect. The deaths Hitler is infamous for are 100% direct. Had he won the war the death toll from the Holocaust would have been almost unimaginable. A huge portion of the world would have fallen under Nazi rule and with a greater amount of resources and the lack of any major powers to contest the Nazis it's likely that the killing would have become far more widespread and efficient. Those Jews that escaped Nazi grasp during the war probably would not have made it. Many survived only because the Nazis could not reach them. As history shows after the war the Russians were mostly contained. While they gained new territory and the Cold War ensued Stalin did not make any major attempts to conquer Europe like Hitler had. I'd argue that he was not even capable as his paranoia made him incredibly ineffective at this point.

Stalin and Hitler were two absolutely horrible people. It's hard to say if one was really more evil than the other, but Hitler was far more dangerous and more effective. No other man in the history of the world has been able to execute a plan of mass murder with such efficiency and on such a massive scale.