Colonialism, being a very important part of European expansion, was the driving force behind many Western actions during the 17-1800s. Europe took it as a moral goal to "civilize" the native peoples of Africa, not just introducing them to technology but forcing them to abandon their heritage and belief systems. This caused intense contention between pro- and anti-colonialists, and of course, between the native peoples and European expansionists. The ivory trade, which is the business at the center of the novel, was more a selfish endeavor, deliberately making money off ivory without care for the future of the elephants or the natives.
"The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much. What redeems it is the idea only. An idea at the back of it; not a sentimental pretense but an idea; and an unselfish belief in the idea -- something you can set up, and bow down before, and offer a sacrifice to...."
(Conrad, Heart of Darkness, gutenberg.org)
It is clear that Marlow no longer believes in colonialism, and so he would understand the push-back from native and indigenous peoples. Although anti-European sentiment does not stem singly from Africa, the pushback against Europe was based mainly in colonialist ventures of this type. Africa was a typical example in that the European atrocities continued until they became too public to ignore, after which Europe was mostly forced out. Continuing memory and resentment from that era can be passed down by generation, with parents wishing their children to not forget how they were treated, and so the colonialist attitudes of the 17-1800s set the stage for general anti-European and anti-Western sentiments.