Marlow starts the story as an idealist, believing in the mission of European Imperialism and in his own destiny to pilot a steamboat, bringing goods and civilization to and from Africa. However, as he ventures deeper into the jungle, he sees madness and atrocities committed by Europeans and natives alike; his realization comes after he meets Kurtz and sees the insanity firsthand:
I felt an intolerable weight oppressing my breast, the smell of the damp earth, the unseen presence of victorious corruption, the darkness of an impenetrable night...
(Conrad, Heart of Darkness, eNotes eText)
It is the immediacy of Kurtz's actions, both shown and heard, that drives Marlow to his epiphany; Man has weakness and brutality in his heart, and without social stigma and judgement it can break free and commit atrocities without concern for consequences. Kurtz shows Marlow that Man is not meant to rule others without balances and advice; Marlow realizes that he too would suffer the slow decline into madness if he were to remain in Africa. In Kurtz's grand plans and insane speeches, Marlow sees his own idealism taken to the opposite extreme, and is repulsed by it.