The numbers I remember from when I was more into History:
* Hitler is responsible at least for 10 million to 15 million deaths.
* Stalin from 15 million to 20 million.
* Mao from 40 million to 70 million.
* Pol Pot from 2 million to 3 million.
* Kim Il Sung for over a million.
There are of course others, but I don't remember numbers for them.
I'd put Hitler much higher, though. That figure must be excluding a lot of war deaths. For non-war deaths, there are 6 million Jews, maybe 3 million non-Jewish Soviet citizens, 3 million non-Jewish Poles, plus a bunch of other groups with smaller numbers. Taking just those big ones, that gets us to 12 million non-war deaths.
But surely we should count at least some of the war dead. Deciding what to attribute to who is very subjective. Starting with the strongest case, 3 million Soviet POWs were killed in captivity, hard to blame anyone else for that. That's up to 15+ million. There's a good case for including Allied military deaths in the European theater, since they were killed by Axis forces. The vast majority of those are the Soviets, which accounts for another 5-8 million (not double-counting the POWs killed). I'd also include Axis military deaths under the general principle that you get credit for what happens when you start a war. That's another 5-6 million. That puts us at 25-37 million.
Then you can get really fuzzy. There's many millions of Soviet citizens who starved due to wartime disruptions, do they count? There's around a million German civilians killed during the war, or died in the immediate aftermath due to the Allies, do they count?
From a quick search, the lowest estimates for Stalin are about 6 million, and Mao about 15 million. Are you saying Hitler killed less than 6 million? Even 15 million would be way too low.