Arguably the worst act of colonialism in history were up to 10 million Congolese people died in 2 decades.
Main Sources:
Red Rubber - ED Morel
Five Years With The Congo Cannibals - H Ward
Leopold to lumumba: a history of the belgian congo, 1877-1960 - G Martelli
King Leopold's ghost : a story of greed, terror, and heroism in colonial Africa - A Hochschild
https://www.britannica.com/place/Congo-Free-State
https://www.theguardian.com/world/2010/jun/30/congo-belgium-brussels-kinshasa