The first example of absolute insanity or madness that I got the greatest laugh from was a statement at the end of the book by Kurtz's intended:
'And his example,' she whispered to herself. 'Men looked up to him--his goodness shone in every act. His example...(Conrad, 76)'
It is an amazing statement. She is obviously week of the mind and unwilling to accept what Kurtz was and capable of. Secondly, the Harlequin or the Russian who works for Kurtz and is kind of a fool for Kurtz. The Russian was nearly killed by Kurtz, yet he idolizes him and tells Marlow the following:
'You don't know how such a life tries a man like Kurtz,' cried Kurtz's last disciple. 'Well, and you?' (says Marlow) 'I! I! I am a simple man. I have no great thoughts. I want nothing from anybody. How can you compare me to...(58)?'
The Russian probably from the stress of his situation has lost his bearings. He is unable to see that Kurtz has harmed him, has killed and robbed the jungle.
With these characters and Marlow himself, I think Conrad was trying to say that even when people commit unspeakable acts or see the unspeakable they somehow or someway justify these events. The Russian by thinking that he himself is a small man and Kurtz a great man. Kurtz's intended justifies it by seeing his actions as genius as well. Both completely ignore the real result of the Genius. It could probably be argued that both suffered post traumatic stress and in order to deal with it they condoned it out of a need for survival. They may have even blocked some of it from their mind at least in the context of how it actually happened. I believe this is what Conrad states many times with how all the people seem to miss what has been done or at least look past the unholy to get at what they need.