Actually, that dubious distinction probably goes to either Pol Pot (another fellow toiler), or, more likely, the Hutus in Rwanda.
Maybe ol' Pol, but I don't think the Hutus made it. They are estimated to have killed about half a mil of the Tutsis in about half a year, if memory serves. Hitler murdered six million Jews and six million gentiles, not counting 20 million Soviets he killed in the war (I'm not getting into the seven million Gerry soldiers, since so many of them were glad to die for the Vaterland.)