Before we talk about humanism in conjunction with Joseph Conrad's Heart of Darkness, it's helpful to define the term "humanism" itself. Humanism can refer to a variety of things, is generally a system of beliefs (a philosophical outlook, really), that celebrates humans as rational and ethical beings, values the pursuit of knowledge and the use of reason, and regards all humans as beings possessing inherent value and/or dignity. In short, humanism sees human beings as inherently good and valuable, and emphasizes human experience and knowledge as important.
Now, let's see how humanism becomes an illusion in Conrad's novel. To start, it's important to remember Kurtz began as something of a humanist, as he claimed his acts among the natives of the Congo had the potential to accomplish unparalleled good for all parties involved. In that case, it's apparent Kurtz initially valued the humanist pursuit of knowledge and believed in the inherent goodness of the human race. It's clear, however, that the humanist mission with which Kurtz began the novel gradually devolves into chaos. Rather than respect the natives as dignified humans, Kurtz rules over them as an autocrat and uses them as means to an end (when he isn't killing them and sticking their heads on stakes, that is). Furthermore, Kurtz ceases to base his activities on reason and rational thought, choosing instead to set himself up as an all-powerful god. Thus, it would appear that, within colonialism's African empire, humanism dissolves and is replaced by greed, fanaticism, and unparalleled cruelty and oppression.
One of the most famous lines from Conrad's Heart of Darkness is spoken by the narrator, Marlow, who states, "All Europe contributed to the making of Kurtz." While Europe enjoyed what has come to be called the Age of Enlightenment, constituting roughly most of the 18th century, it disgorged from its shores numerous explorers, merchants, adventurers, religious representatives and scoundrels to the "savage" corners of the world.
Many of those who departed Europe went in the name of European civilization, imbued with the notion that they were delivering to "uncivilized" parts of the world the best and the brightest representatives of the newly-minted humanist fervor, an intellectual flowering that had resulted in loosening the shackles of religious dogma and the constraints of centuries of tradition.
Kurtz represented this notion; in a letter to the officers of the ivory company in Brussels prior to his embarking for Africa, he pronounced his humanist intention that "By the simple exercise of our will we can exert a power for good practically unbounded..." The story devolves, however, into a nightmare of exploitation, murder and madness, as Kurtz surrenders to the tyrannical darkness within his heart and comes to view the Africans he encounters as savages, exhorting his European cohorts to "Exterminate all the brutes!"