I believe Natal was the closest Mark Twain came to the Belgian Congo. His King "Leopold's Soliloquy" is a work he can be proud of. It is probably the earliest work on modern human rights. BUT. Back in that period with no census and no accurate centralised record keeping any kind of mortality figure has to be treated with scepticism.
Using the sentence "Hochshild more or less established that the ten million was correct" may convince you but the first census of the Congolese population was made in 1924, there is a consensus among historians that accurate predictions of the population fall or number of deaths is impossible. There is no denying the figure was in the millions but quotes of 6,10 or 13 million cannot be reliably established. The crime has been established, the death toll was high,in the millions, Just how high simply distracts from the main issue- that it was a crime against humanity. Louis and Stengers state that population figures at the start of Leopold's control are only "wild guesses".
Another factor reducing population was disease. Colonialism also played a part in the mortality from sickness.Diseases imported by Arab traders, European colonists and African porters ravaged the Congolese population and "greatly exceeded" the numbers killed by violence. Smallpox, sleeping sickness, amoebic dysentery, venereal diseases (especially syphilis and gonorrhea), and swine influenza were particularly severe. Sleeping sickness, in particular, was "epidemic in large areas" of the Congo and had a high mortality rate. In 1901 alone, it is estimated that as many as 500,000 Congolese died from sleeping sickness. It goes without saying that malaria is endemic in the area including cerebral malaria.

Even today it is estimated severe malaria kills approximately one million children in Sub-Saharan Africa alone.