Yeppers, Hitler was third most monstrous in sheer numbers. In terms of percentage of "available" population (i.e. potential victims within his power), Pol Pot may have been the most monstrous.
Outside of the sanguinary realm of socialism, the worst monster may have been Leopold II, estimates of whose death toll from his private company's exploitation of Africans range from 1 million to 15 million (they weren't counting, there was no census or high quality guess of the population of Africans under his control, and disease epidemics such as small pox and trypanosomiasis complicate guesstimation). He may be a challenger to Pol Pot in the arena of population percentages.