Simple literary archetypes may be found in Kurtz and Marlow. Kurtz is villain, evil incarnate, capable of the worst kind of human degradation. Marlow is hero, one who undertakes a perilous journey; faces death; encounters, resists, and overpowers evil (Kurtz); and survives destruction. He lives to tell the tale, and he plays the part of the hero again when he protects Kurtz's fiancee from the horrible truth.
On a much deeper level, Jungian archetypes can be found in the novel, primarily that of "The Shadow," the darkness in each human being that remains mostly hidden until called forth. It embodies savagery and evil, promotes chaos and conflict, and seeks to destroy. It haunts human consciousness, and although it may not be recognized in the self, it is recognized in others.
This archetype is, of course, most notably evident in Kurtz, but it is also found in all the forces of destruction identified in the novel, especially those of Colonialism with its inherent greed, cruelty, exploitation, and racism. As Marlow journeys physically into the interior of the dark African continent, he makes another journey into the dark heart of man's basic nature--and barely escapes with his life and his own soul.