Despite the worst dictators in history that come to mind, Leopold II, the king of Belgium between 1865-1909, often doesn't ring a bell. But during the time he was personally given ownership of what is now the Democratic Republic of Congo, was ironically named "The Congo Free State". During his ownership of the land and by extension, the inhabitants within it between 1885 to 1908. He put them in forced labor, and it is estimated that roughly half the population died from overwork, torture, and just straight out murder, along with mutilations whenever their quotas weren't met(the key resource being rubber). The casualties estimates vary very widely depending on who you're talking to, the highest estimate goes up to 15 million, others as low as 1 million, simply because there wasn't a consensus at the time.

The first consensus was done by the Belgian government in 1924, and estimated a population of about 10 million, which one historian from using local sources, came to the conclusion the most likely death toll is probably around 10 million, but no matter if it's 1 million or 15 million it was nonetheless mass murder, exploitation and mistreatment of a people on a mass scale, and truly was a dark part of recent history. If anything this is probably among the worse cases of imperialism alongside the Armenian genocide by the Ottomans, the killing of Chinese by the Japanese empire, or the 1.5 million deaths in Central Asia by Russia, or the genocide of the Herero people in Namibia by the German empire, the US is the Philippines, France in Algeria(it wasn't called land of a million martyrs for nothing), the Italians in Libya, you get the idea.

But anyway, here's a video giving an overview of King Leopold II's exploitation of the so called "Congo Free State"