One important point is that Leopold II's Congo Free State (which was very much the den of horrors you saw described--see Hochschild's King Leopold's Ghost for probably the most accessible recounting of the tale, but there are other more academic ones) was not the Belgian Congo--the two terms refer to separate, though sequential, periods of rule. The conditions of Leopold's personal / company rule over the Congo basin were so bad that the international community and the Belgian public demanded that the Belgian government prise the colony away from Leopold and take a more "moral" stewardship, which they did in 1908. Casement's report for the British government is out there, as is the international commission report which Leopold sent in to whitewash everything--but which came back unable to do anything but damn his rule. What E. D. Morel wrote in Red Rubber is sensational, but in this case it was quite true. Hostage-taking, dismemberment, virtual enslavement, and capricious acts of wanton violence were quite widespread in the service of getting rubber as quickly as possible while it was still valuable. Once SE Asian and other plantations came up to speed, the profit margin dwindled.

Under Belgian colonial governance proper, the Congo was still managed quite badly--but it wasn't murderous, and there was some modicum of accountability. But they also did not do anything significant to promote social or economic development in the territory (beyond extraction of minerals via the Union Muniere), even of the limited degree in British and French colonies (hell, even Portuguese).

[edit: For a lot of actions in the Congo Free State, we have only eyewitness accounts and oral reports transcribed--one thing Leopold II did when it became clear the Belgian government was going to take the colony, is order the destruction of any records they could lay their hands on. So the record is fragmentary in a lot of places, but what does survive is gruesome.]