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.independent.co.uk/news/long_reads/belgiums-genocidal-colonial-legacy-haunts-the-country-s-future-a7984191.html

https://www.npr.org/2018/09/26/649600217/where-human-zoos-once-stood-a-belgian-museum-now-faces-its-colonial-past

https://www.telegraph.co.uk/news/worldnews/europe/belgium/1458736/Belgian-fury-at-film-on-Leopolds-Congo-terror.html

https://www.britannica.com/place/Congo-Free-State

https://www.theguardian.com/world/2010/jun/30/congo-belgium-brussels-kinshasa