The divide of the races into black and white adds complexity to Conrad’s theme of light versus darkness and good versus evil. The conventional use of white as good and black as evil is clearly challenged when we view it through the lens of race, particularly when we see white men brutally subjugating and forcing black Africans into hard labor simply for profit. The Europeans justify their mistreatment of the Africans with claims of "spreading civilization," of helping Africans become "enlightened." This, in itself, is a form of prejudice – a denial of the Africans’ traditional lifestyle and culture.
Despite white Europe’s good intentions towards the Africans and their desire to "civilize" the black man, imperialism proves to be a brutal and callous victimization of the native Africans for the sole purpose of maximizing profits.
Despite Kurtz’s brutality, he treats the Africans more civilly and more as equals than the majority of the other white European characters (like the accountant, the manager, and even Marlow). This is why Marlow sees him as the lesser of two evils.