A few points.
Hitler killed 6 million Jews and five million other people (Gays, communists, Roma, Sinti, the disabled, etc.) Not to mention triggering a war that killed 70 million worldwide.
I think you meant to say Stalin killed 20,000,000 of his own people, but those numbers are largely bullshit, and the only way you get close to that is if you throw in famines like the Holodomor in Ukraine. Kind of like the Bircher crazies screamed about Mao killing 60 million, but you only get those numbers if you include the famine of 1959-62. (Forgetting that Famines were regular events in China, Russia, and pretty much the rest of the world prior to the Communists.)
Here's the problem with the "20 million" figure. Census figures from the USSR just don't hold it up.
The population of the USSR in 1926 was 147 million. By 1939, the eve of the war, despite all the purges and famines, the population increased to 170 million. By 1959, despite the War, but including the territories the USSR annexed, the population increased to 208 million.
en.wikipedia.org
The numbers, just don't add up.
Not to say Stalin wasn't a ruthless bastard (he was), that he did a lot of damage to the USSR (dumb collectivization that led to famines, purging his best officers so no one knew what they were doing when Hitler invaded).