There's method in this madness: Kurtz has gone from noble conqueror to deranged slaver because his power and greed have been totally unchecked. Ergo, if you want to stay sane, don't swoop into an African village and start passing yourself off as a god. Point taken. But that's where things start to get hazy. Is madness is just another name for imperialism—the idea that white men can swoop into Africa and claim it for themselves? Or is madness what happens when civilization tries to conquer the wilderness?
Isolation and life in the wilderness cause 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 always present.