George Washington Williams witnessed the situation in Congo, documented what he witnessed and exposed it to the world.

An African-American Christian activist named George Washington Williams traveled to Congo as a missionary. When he arrived at Congo he was furious and disheartened. He witnessed the Force Publique burning the locals’ towns, stealing their property and enslaving both children and women. Additionally, Leopold had not spent any money for educational purposes or to  industrialize the country. The Congolese could only eat rice twice a day, lacked water, were exposed to extreme weather conditions, and slept in areas that were unsanitary. Soldiers promised the Congolese that they would get paid, but instead they were given handkerchiefs and gin. During the 13 years in which the Congo was colonized, only one white man was killed by the locals and two white men were killed in the Congo, yet [how many were killed by the Belgians? – you need to make a contrast so we know why you put this]. Williams was very upset by all he observed and wrote an open letter on July 18, 1890 to Leopold. He intended for European leaders to read it and take action. He also sent a  letter to U.S. president Benjamin Harrison, encouraging action. Also, Williams wanted to tell Leopold directly but since it was an open letter, it was shown for publicity.

Evidence 

George Washington Williams wrote a open letter to King Leopold of Belgium on his study and inspection of what he witnessed at the Congo:

“When I arrived in the Congo, I naturally sought for the results of the brilliant programme: “fostering care”, “benevolent enterprise”, an “honest and practical effort” to increase the knowledge of the natives “and secure their welfare”. 

“The African soldiers and labourers of your Majesty’s Government fare worse than the whites, because they have poorer quarters, quite as bad as those of the natives; and in the sheds, called hospitals, they languish upon a bed of bamboo poles without blankets, pillows or any food different from that served to them when well, rice and fish.”

“Upon the arrival of the survivors in the Congo they are set to work as labourers at one shilling a day; as soldiers they are promised sixteen shillings per month, in English money, but are usually paid off in cheap handkerchiefs and poisonous gin. The cruel and unjust treatment to which these people are subjected breaks the spirits of many of them, makes them distrust and despise your Majesty’s Government. They are enemies, not patriots.”

Williams wrote the letter to Leopold II for the reason of accusing him for treating the Congolese brutally. Williams was trying to appeal to Leopold’s reference to how Congo was “benevolent enterprise” and “honest and practical effort.” But in reality, it contradicted what he witnessed when he arrived in Congo.

"(1890) George Washington Williams's Open Letter to King Leopold on the Congo." Blackpast.org, Accessed 1 Jan. 1970.