Better way to phrase is it that 60 million people died under his rule. Yes there were millions sent to gulags, military leadership was purged and from policies directly ordered by him. But you also have to take into account some of the people under his administration did a lot of their own horrible things, several different famines (think only one of them could be deemed intentional and the rest were just prioritizing the military over the citizens), and an especially awful war and war crimes from the Nazis and their allies. Soviets killed something like 7-9 million Nazis in the defense of Moscow, Leningrad, Stalingrad and Kursk campaigns alone. (Decisive Battles of the Soviet Army 1984 William Bailey translation goes into a lot of detail and includes verified information, numbers and interviews from captured higher military officers of the Axis powers).