What were negative effects of imperialism on the Congo?

Among other things, the Congo was run essentially as a personal fiefdom by the Belgian King Leopold I, who used a great deal of brutality in an effort to essentially extract as much wealth as possible, primarily in the form of rubber, from the colony. Coercive tactics such as mutilation and execution were commonly used in order to ensure compliance. See Hochschild, King Leopold's Ghost (1998).