Modernism loosely coincided with a widespread movement toward commercialism, a new kind of nationalism and the end of imperialism world-wide (due to its replacement by a global capitalist system). The dual emphasis on a personal relationship with history and the notion of subjective existence serves to characterize modernism and characterizes Conrad's work as well.
Imperialism (commercial interests at a national level) is central to the text of Heart of Darkness, and critical to understanding Kurtz in particular. Kurtz sets out to abandon his professional role and go beyond commercial interests. His efforts are a confusion of motives relating to an idea that, on the one hand, society should not be able to dictate morality to a superior individual and, on the other hand, there is no individual without society.
Kurtz' dissolution grows from this division which defines the "horror" he experiences. By adandoning both his commerical role and the morality of society, Kurtz becomes a man without a country.
As he is sent into the Congo for national commercial interests, he essentially negates himself in his efforts, as is evidenced by his change of heart regarding the natives ("Exterminate all the brutes!").
Additionally, in keeping with modernism's fixation on a personal relationship with history, Kurtz can be seen to have lost himself by severing his identity from his past. In this light, to become wholly new is, effectively, to court insanity. Similar dynamics characterize Darl in Faulkner's As I Lay Dying and Benjy and Quentin in The Sound and the Fury.
As nationalism takes on new bureaucratic form in the early 20th century, the individual becomes an economic unit and a tool of production. Rejecting this notion of self, the individual clings to a romantic era view of self.
In Conrad's Lord Jim, we have an example of this posture. As Jim agonizes over his failures, he presents a view of himself as something other than a functionary. He sees himself as a "romantic" kind of man.
His remorse is an index of his romantic view of himself and of the individual’s power over his own impulses. (Cyclopedia of World Authors, eNotes)
In Heart of Darkness, Marlow stands as the "modern man" witnessing his own future in Kurtz should he too choose to reject the changes that modernity brings (as he is clearly tempted to do). Kurtz rejects his role in the corporation and, lacking a place in this body, becomes placeless entirely; unmoored practically and psychologically. Marlow, given this example, is able to refrain from going so far toward insisting on the rejection of society.
Concerned with the "progress" in industry, commerce, imperialism and nationalism, the movement known as modernism was closely connected to a negative view of what these movements represented for manking. Conrad's work participated in the same doubts.
Much of Conrad’s fiction...reflects his fatalistic view of history and his skepticism about the possibilities of progress in human affairs. (Cyclopedia of World Authors, eNotes)
If imperialism can be taken as an expression of the symbolic and practical nature of society at the turn of the century, we can argue that Conrad's work stands as a commentary on imperialism and as a direct precursor to modernism in this regard. Man's view of himself and his place in the "world-order" is deeply questioned in Conrad's work, especially as it is seen to be changing.
Imperialism can be understood in this context as a commercial insistence on the new perspective on humanity that industrial progress and bureaucracy have wrought.