The question of how and why one goes mad in the interior pervades the novel. Conrad suggests that the white man’s fear of Africa’s unexplored heart, her ‘savage’ people, and the crew’s greedy internal power struggle thrust Marlow (and before him, Kurtz) towards madness. When one becomes so far removed from society’s mores and restrictions, good and evil become relative terms. When these moral boundaries begin fading, Conrad suggests, man loses the sense of where he stands in the great moral struggle. Having lost this foundation, it is a short step to losing one’s mind.
Isolation and life in the wilderness is the sole cause of Kurtz’s madness; in other words, there is something inherently madness-inducing about the African interior.
One of Conrad’s main messages is that madness is not caused specifically by living in the wilderness, but that the seeds of madness – ambition, obsession, and greed – are already extant in a man before he journeys into the interior.