Who is the protagonist and who is the antagonist of the story?
Because most of our attention is focused on Marlow, he is the protagonist. Determining the antagonist is more difficult because he is not in conflict with another specific character as a protagonist usually is. Instead, what he recounts to his friends on board the Nellie is a scathing account of the devastation wrought by the white Europeans engaged in ivory trade in Africa. Rather than being the "emissary of trade" that Marlow's aunt believes him and other white Europeans to be, those seeking ivory are destroying the "savages" and Africa in search of profit. Marlow goes to Africa so that he can pilot a steamboat up the Congo. Only one white is worthy of his admiration, the Boilermaker, because this man is meticulous and hard-working while all others allow disorder, cruel treatment of the Africans, and laziness among themselves. In fact, Marlow also admires the cannibals because they possess restraint, a quality entirely missing in the white Europeans. Although Marlow is not in direct conflict with these men, Conrad uses them to be symbolic of the damage Europeans are thoughtlessly causing in Africa; therefore, the white Europeans are the antagonists. Perhaps Kurtz is the most important of these. On another level, you can also identify a man vs. self conflict in which Marlow plays both parts.