While it has proven difficult to accurately estimate the pre-colonial population and the amount by which it changed under the Congo Free State, estimates for the Congolese population decline during Leopold's rule range from 1 million to 15 million.
Noobish kdr. Max 10 millions deaths, somewhere between a French general during WWI (not counting German casualties) and minor paranoid dictators. Too much time larping as a psychopathic torturer, not enough time getting the job done. Mao killed several million people on a good mood, that's professionalism.