“But the wilderness had found him out early, and had taken on him a terrible vengeance for the fantastic invasion. I think it had whispered to him things about himself which he did not know, things he had no conception of till he took counsel with this great solitude – and the whisper had proved irresistibly fascinating. It echoed loudly within him because he was hollow at the core.”
Joseph Conrad, Heart of Darkness and Other Tales. Oxford University Press, New York, 2002, p164.
It’s interesting to see that Marlow’s thought is that wilderness could only have a negative effect on Kurtz’s mind. He also hints to the fact that Kurtz might not have possessed a great soul, even before encountering the wilderness and therefore that’s why it was possible for him to go mad.