If we go purely by deathrates then we should pick communist china or the soviet union.
If we go by barbarity the mongol empire, the timurids, and the japanese empire (talking the one from ww2) are all good contenders.
Imperial japan was so bad that the nazi ambassador to china in nanking did his best to save as many civilians as he could out of sheer disgust for their behavior.
imperial japanese soldiers used to bayonet babies and do beheading contests for fun.
literally, their newspapers had them even keeping score over who could do more beheadings.
they melted people, literally ate people on one occupied island, they did multiple genocides, they tested gasses.
Imperial japan is what I consider the worst by barbarity.