I’d say Stalin. His purges and famines killed tens of millions, and they stopped once he died and was replaced.
He also decimated the officers corp in the Russian military, so when Hitler attacked the Soviets weren’t as capable of fighting back. Who knows what effect that had on the duration of WW2. But maybe it had some good too. A Soviet Russia that fully conquered Germany and Japan wouldn’t be too good either.
He was supposedly planning a massive pogrom of hte Jews and a nuclear strike on the US, and was poisoned by Beria (his security chief) in 1953. So tens of millions more dead. Probably the one redeeming thing Beria did in his whole life.
He probably helped ruin communism as a palatable ideology (him, Mao, Pol Pot, etc), which may have set back worker and labor rights all over the globe.