I would probably go with King Leopold II. Several hundred thousand Congolese were massacred under his ownership of the Congo Free State, and his regime was so brutal in its regimentation of rubber extraction that many of its residents were subjected to bodily mutilation among other atrocities. There was public outcry following the exposure of conditions under his leadership, even among defenders of colonialism, and he was eventually forced to hand it over to the Belgian government who administered it as a traditional colony, but did not provide full autonomy to the Congolese population and continued to oppress the population and work against its independence and self sufficiency well into the administration of Congo's first democratically elected president, Patrice Lumumba