What led king leopold ii to lose control of lands in the congo? how did humanitarians react?

Leopold claimed that his primary motive in establishing a colony was to abolish the slave trade and promote Christianity. 
But as it turned out, Leopold forced the people in that region to do hard labour for his plantations. Humanists rected by demanding changes to the government and eventually leopold's operation was taken over by the Government.

Democracy and darwinism would be my answer.