"Why would they kill us, they need us to work for them"
They don't. They didn't need us back then, and they sure don't need us now. They spend all their money to sterilize us and kill our children, and call it philanthropy.
They don't care about us, they want us dead. They hate us and don't see us as humans. The descendants of those who did this still rule the planet, and talk about how they want to be reincarnated as viruses. The descendants of those who put cancer viruses in our vaccines support China's one child's policy and brag about how they love Mao. They put the biggest murders into power.
Native labourers who failed to meet rubber collection quotas were often punished by having their hands cut off.
One junior white officer described a raid to punish a village that had protested. The white officer in command "ordered us to cut off the heads of the men and hang them on the village palisades ... and to hang the women and the children on the palisade in the form of a cross." After seeing a native killed for the first time, a Danish missionary wrote: "The soldier said 'Don't take this to heart so much. They kill us if we don't bring the rubber. The Commissioner has promised us if we have plenty of hands he will shorten our service.'" In Forbath's words:
The baskets of severed hands, set down at the feet of the European post commanders, became the symbol of the Congo Free State. ... The collection of hands became an end in itself. Force Publique soldiers brought them to the stations in place of rubber; they even went out to harvest them instead of rubber... They became a sort of currency. They came to be used to make up for shortfalls in rubber quotas, to replace... the people who were demanded for the forced labour gangs; and the Force Publique soldiers were paid their bonuses on the basis of how many hands they collected.
In theory, each right hand proved a killing. In practice, soldiers sometimes "cheated" by simply cutting off the hand and leaving the victim to live or die. More than a few survivors later said that they had lived through a massacre by acting dead, not moving even when their hands were severed, and waiting till the soldiers left before seeking help. In some instances a soldier could shorten his service term by bringing more hands than the other soldiers, which led to widespread mutilations and dismemberment.
 Death toll
Estimates of the total death toll vary considerably. The reduction of the population of the Congo was noted by all who have compared the country at the beginning of the colonial rule and the beginning of the 20th century. Estimates of observers of the time, as well as modern scholars (most authoritatively Jan Vansina, professor emeritus of history and anthropology at the University of Wisconsin), show that the population halved during this period.
According to Irish diplomat Roger Casement, this depopulation had four main causes: "indiscriminate war", starvation, reduction of births and diseases. Sleeping sickness ravaged the country and was used by the regime to account for demographic decrease. Opponents of King Leopold's rule stated, however, that the administration itself was to be considered responsible for the spreading of this dreadful epidemic. One of the greatest specialists on sleeping sickness, P.G. Janssens, Professor at the Ghent University, wrote:
It seems reasonable to admit the existence on the territories of the Congo Free State, of French Congo and Angola of a certain number of permanent sources that have been put again in activity by the brutal changement of ancestral conditions and ways of life that has accompanied the accelerated occupation of the territories.
In the absence of a census (the first was taken in 1924), it is even more difficult to quantify the population loss of the period. Casement's 1904 report set it at 3 million for just twelve of the twenty years Leopold's regime lasted; Forbath, at least 5 million; Adam Hochschild, and Isidore Ndaywel è Nziem, 10 million; the Encyclopædia Britannica. and Fredric Wertham's 1966 book "A Sign For Cain: An Exploration of Human Violence" estimate that the population of the Congo dropped from 30 million to 8 and 8.5 million, respectively, in that period. Similarly, the New York Times reports that "Under the reign of terror instituted by King Leopold II of Belgium (who ran the Congo Free State as his personal fief from 1885 to 1908), the population of the Congo was reduced by half -- as many as 8 million Africans." 
In 1900 Africa had between 90 million (African Studies Review 49.1 (2006) 179-181) and 133 million people (World Population Prospects: The 2006 Revision).