A lot really depends on how/what you view the "killing" as. If you take all deaths "equally", Mao had double / triple (depending on what numbers you use, it's roughly estimated to be between 50-80million) the amount of people killed than Hitler & Stalin almost double (roughly 40million but estimates are up to 60 million). The line I've heard other historians use is "In the league tables of horror, Mao comes top, then Stalin and Hitler."
If we are specifically talking about killing people specifically on "racial superiority " then the closest comparison I would make is the Ottoman Genocide in Armenia around World War 1.
My history thesis (for American University) was an argument on why Stalin's policies with collectivization were tantamount to genocide because the policies were failures and left to fail leading to all of the deaths --which Mao made similar policies. (IDK why, but this topic is one of my most favorites to study in history)
I listen to Real Dictators podcast, which I really enjoy -- they talk about the rise/fall of dictators around the world.