History tends to spend less effort condemning the actions of those who don't start off with dramatic televised speeches about the people they want to eliminate and sign off with a World War involving almost every country worldwide.
Still, at the time, the actions of Leopold's men in the Congo were considered so evil that Leopold was forced to relinquish control of the Congo by his own government, and you won't find anyone willing to argue with a straight face[1] that the Congo Free State represented anything other than the worst excesses of colonialism.... even at the time[2]. If you'd released an info graphic depicting black people with their hands severed and white people counting cash 100 years ago, educated viewers would have got the reference. And probably written to complain that the administration of the Congo Free State was a far more complex and nuanced issue than simply "good guys" and "bad guys".
[1]Mark Twain did so with heavy irony.
[2]Leopold was even condemned by the British, whose monarch was a close relative and whose government was a Belgian ally with imperial possessions of their own covering half the globe, and an army fresh from conducting policies of mass-internment of civilians in the Boer War
Because we don't rank evilness based on the casualties, we rank it by how close it is to us. The holocaust impacted modern western civilization (there still are people today who lived the thing), so it is regarded as the most horrible thing ever in the west.
