What is Marlow's attitude toward the African people he encounters on his trip up the Congo?

I think this changes as the story progresses. Initially he saw them as alien and strange. By the end of the novel he saw them as human beings who have had their souls sucked out of them by the colonial invasion.

Some would say that Asian's answer is true. For some reason though, I feel that Conrad never truly took the time to give any kind of humanity to the African natives.

As someone else puts it "It can be argued that Heart of Darkness participates in an oppression of nonwhites that is much more sinister and much harder to remedy than the open abuses of Kurtz or the Company’s men. Africans become for Marlow a mere backdrop, a human screen against which he can play out his philosophical and existential struggles. Their existence and their exoticism enable his self-contemplation. This kind of dehumanization is harder to identify than colonial violence or open racism. While Heart of Darkness offers a powerful condemnation of the hypocritical operations of imperialism, it also presents a set of issues surrounding race that is ultimately troubling." I just could not say this any better so I quoted it.