20 million is a vast overestimation for how many deaths he was responsible for.

The actual number of executions and other deaths resulting from the purge is about 700000, many of those were probably actual criminals. The number of deaths under GULAG during his leadership is uncertain, but definitely under a million, many also actual criminals. About 4-6 million died due to a mismanaged famine triggered in some part by his idiotic agricultural policies, but this is on the edge of "responsible for". That's about it when it comes to peacetime deaths he was involved in.

Was it bad? Sure, it was fucked up, he's the second worst leader of USSR after Gorbachev. But he's not as bad as many western propaganda sources will have you believe, and I'd argue trump is worse overall.