I think an interesting take on this is that one of the main reason Hitler couldn't kill as many as Stalin was because unlike Stalin (or Mao for that matter), we actually stopped Hitler midway through his efforts.
Because other things were going on, nobody interfered to put a hard stop to what Stalin and Mao were doing, while Hitler lost the war he was fighting and basically had to unconditionally surrender and stop his efforts dead. We'll never really know what would've happened if Hitler could continue doing what he was doing, but saying Stalin is worse because he got to finish what he started doesn't seem quite right.
Also, Skye Reynolds posted...
It's not just a matter of how many died, but also why they died.
Hitler's motivation was racial hatred and he was so fervent in his hatred that he pulled soldiers from the front lines to hunt down Jews in neighboring countries. Genocide was his top priority; even over winning the war itself.
Stalin was evil, and murdered more people over a greater span of time, but Hitler's top priority was ridding the world of an ethnic group he despised. And that's a heinous as anyone could possibly be. Even Stalin would have prioritized the war over personal prejudices.
The gas chamber itself largely came about because the people who had previously been tasked with genocide had a high suicide rate. They needed to make death impersonal and so they created a system for which one would not have to look the victims in the eyes. Murderous scumbag human beings who swallowed the Nazi rhetoric wholesale were killing themselves because they were being forced to be more evil than most human beings are capable of and a new system had to be devised to make death impersonal for the murdering party. And the only motivation was ethnic hatred.