They didn't kill all these people, you have some hiperinflated casualties there.
Hitler caused the "unlawful" (Our of the rules of war) death of ~12 millions during WW2, while Stalin killed a few thousand of political prisioners and caused the death of ~7 million of minorities and criminals.
WW2 can be ultimately attributed to the war economy of Hitler but he cannot be made directly guilty, but it's death tool was more than 50 million.
Stalin didn't have anything near dictatorial power. That's attributable more to cult of personality than historical fact. Legislative authority rested in the Supreme Soviet, which consisted of approx. 2000 members with an equal vote. Even the Central Committee always had more than 33 people.
There's no denying that Stalin as a figurehead had a great deal of political swing, but the USSR could be more accurately described as a dictatorship of 2250 democratically elected people. The Chairmen of the Presidium of the Supreme Soviet of the USSR from 1938 to 1953 were Mikhail Kalinin and Nikolay Shvernik. As General Secretary, Stalin was subject to re-elections and oversight by the Supreme Soviet. In fact, during the 17th Congress of the All-Union Communist Party in 1934, Stalin received a few hundred negative votes out of 1961 delegates. Kirov, in fact, received less negative votes and was approached by multiple senior Party members, but reported the incident to Stalin (one of his close friends) and refused the position of General Secretary. Stalin himself attempted to resign three times, one when his wife committed suicide, and his resignation was rejected by the Party on all three occasions.
This comparison overlaps. Russia's casualties in WWII are attributed to both, making this a strange comparison. Realistically, only Germany's casualties (and possibly Austria's) should be attributed to Hitler, not that of every nation involved.
My argument is that by invading Poland, Hitler pushed the world into conflict and as a result, you could attribute the losses of all sides to his action, at least in the European theatre. I can see your point though, and I can see why my point is a bit shaky.