King Leopold's Ghost by Adam Hochschild was an eye opening book about the atrocities committed by Leopold II in what was then Congo Free State. (How they can use the term "Free" is beyond me). To your point, exactly right, Leopold II went beyond Hitler.
That was an evil mofo who has been all but forgotten. The Congo was able to get temporary freedom but the coons plunged them right back into Belgian slavery.
That!! Patrice Lumumba was well on his way to getting freedom for the Congolese, til he was set up. So many people have blood on their hands regarding Lumumba's assassination.