I think that one could make a strong case that Conrad's work becomes more powerful when examined from a Modernist point of view. Given how the novel was published at the turn of the century, it becomes clear that Conrad's work is inserted into a progression of industrialization, imperialism, and growing advancement that reflects a certainty in how nations dominate and control other nations. Marlowe is a part of this certainty and by the end of it, is transformed by it into being more ambiguous about it.
It is in this frame of reference where I think that a Modernist reading of the work illuminates so much from it. If one takes the Woolf definition of Modernism, Conrad's work embodies it in a strong manner: "“All human relations shifted, and when human relations change there is at the same time a change in religion, conduct, politics, and literature.” Conrad's work speaks to how there is a fundamental "shift" in human relations and how the work generates a "shift" in how relations are constructed. The supposed certainty with which European nations colonized the world, in particular areas such as Asia and Africa, is "shifted" in Heart of Darkness. When Kurtz says, "Kill the brutes" and then is reduced to a shell of his former self with his final words of "The Horror," it becomes clear that the novel indicates a shift in how Europeans and Americans were interacting with the world. Through this, one can ascertain that Conrad is making a statement about how there are limits to Western imperialism, a frame of reference that shifts the understanding of the time period. The fact that the novel is told in a frame story where the narrator is also "shifted" helps to embrace another Modernist tendency. Analyzing the work in a Modernist approach allows the individual to understand the modern sense of alienation that was so intrinsic to works of the time period. There is a hollowness evident in the mission, in Kurtz, in Marlowe, and in the very ideas that individuals held to be true. Part of the shift that Conrad is able to accomplish is to effectively question the "absolutes" of order and disorder, sanity and insanity, as well as how hypocrisy is a part of the Modern condition. It is here in which analyzing the work from a Modernist frame of reference illuminates so much of its effectiveness, and thereby might be the best approach to take.