Why is Marlow racist?

Throughout the first part of the novel we see that Marlow is in fact a very racist person. He is seen describing the Africans he encountered in the Congo as "black shadows of disease and starvation, lying confusedly in the greenish gloom." He takes pity to these people but unlike the others that work for The Company he actually realizes that they are in fact people. The Company describes their purpose for being in Africa as a means to bring humanity to the people but they are just killing freely and no one realizes the horror in their true intentions, except for Marlow. Marlow has an encounter with a dying African he sees in the shade he is loitering in. As a generous donation, Marlow gives this man a biscuit he had in his pocket. He describes his action as "finding nothing else to do". He gazes upon the man and realizes that he is wearing white worsted. It startles Marlow to see such a "creature" as he describes them to be wearing such linen from oversees. He also notices other men that he described as being "acute angles", not even able to call them people even though he is able to share his food with them. To contrast, Marlow’s meeting with the white accountant that works for the company shows Marlow's feelings towards white people. The man is well dressed with "snowy trousers" and "white cuffs". Marlow had no problem seeing this white man wearing white attire, but once he saw the white worsted tied around the black man’s neck he questioned the meaning of it. As if he was saying that the black man was not allowed to wear anything white because he is a symbol of darkness and death. Marlow's racism mostly comes from what he has been told by the Company but he sees their true intentions first hand so why is he still so racist?