I'd have probably said Pol Pot.
The Red Emperor was a psychopath with no care for anything but himself and his country's bottom line, but you could at least see some kind of justification on his end where he was attempting to strengthen China and just didn't give a s*** how many people would have to die either by starvation, over-exhaustion or execution of the intelligentsia. Pol Pot meanwhile, while he had nowhere near the death toll didn't give a **** about anyone or anything other than solidifying his own power and would do similar things to what Mao and others did but without any care for whether it was actually sustainable or had any positive impact on his own country. It kept the people down and kept him in power.
Ditto Hitler, yes he was a scumbag but he was still enough of a nationalist that he did care about more than just himself.
It's a close one between Hitler an Mao, the death toll favours Mao in terms of being "more evil", but simultaneously he wasn't picky and didn't scapegoat anyone not belonging to an Aryan ideal... but simultaneously he was slaughtering anyone smart enough to potentially be able to motivate the masses. Both are cases of slaughtering innocents. But I'd still go with Pol Pot over both. A bigger piece of work who, fortunately, was never in as powerful a position as Mao or I don't think it would even be up for debate.