King Leopold II has a different personality than OTL and gets massively interested in civil engineering, in particular dams. When he gains the Congo, he decides that in order to "civilise" the inhabitants, they need a dam on the Congo. Such a dam would bring enormous prestige to Leopold II as well. So King Leopold has plans drawn up to dam the Congo River and use it for hydroelectricity. The workforce is largely people conscripted from various villages in the Congo working in conditions akin to slavery.
Although it would make among the largest power stations in the world, the side-effects would be an insane carbon footprint (from drowning all that rainforest), an increase of malaria and earlier spread of HIV, a likely increase in ethnic tensions and internal conflict in Central Africa, and the potential of the dam being a target for terrorism (although nothing short of a nuclear bomb could possibly break the dam) with extreme consequences. The Congo Dam and the lake it creates would be one of the greatest symbols of colonialism and its impact on Africa.
On the bright side, the fact it generates so much power could be beneficial for the economy, and a canal connecting it to Lake Chad would refill that lake and be a good transport corridor, preventing the real threat of Lake Chad drying out which would have serious consequences. However, you could do both without needing such a massive lake.
Mass genocide, as they have no way to move the millions of people in the area of the lake. Nor have they any place to put the ones that do flee.
The lake would take almost a century to fill IIRC, so few people would drown. The displacement of millions would occur over generations.
I doubt the Belgians would want to make the lake that big, however, a much smaller lake seems possible. On the other hand, while I am certainly not an architect and have no expertise on this subject whatsoever, such a project seems a bit too advanced for the era of European colonialism, especially if a minor nation like Belgium if building it rather than the British and French, who would have more resources at their disposal to be utilized in such a project. Maybe with either US or Soviet backing (I think the DRC used to be a Soviet ally around the time of independence) the dam could be built once the DRC achieves independence, thus turning the nation into a regional economic powerhouse that would have plenty of electricity.
See the Grand Inga Dam project, which goes for a much saner hydro scheme which won't drown the entire area. That would be a highly beneficial project for Congo-Kinshasa.