This beast murdered no less than 10 million people in Kongo.
Adam Hochschild devotes a chapter of his book, King Leopold's Ghost, to the problem of estimating the death toll. He cites several recent lines of investigation, by anthropologist Jan Vansina and others, examining local sources from police records, religious records, oral traditions, genealogies, personal diaries, and "many others", which generally agree with the assessment of the 1919 Belgian government commission: roughly half the population perished during the Free State period.
Since the first official census by the Belgian authorities in 1924 put the population at about 10 million, that implies a rough estimate of 10 million dead.