How does Marlow describe the partially completed report that Kurtz penned before lapsing into his fatal illness? What effect does that report have on Marlow?

Marlow discovered just how twisted Kurtz was. He thinks that Kurtz succumbed to the darkness of a Jungle not intended for Europeans. He does give the official report to "The Society for the Suppression of Savage Customs". Marlow makes sure he rips out the page that says, "Exterminate all the brutes."