It's purposefully left ambiguous. We're not let into Marlow's thoughts. But we can speculate. Perhaps Marlow thinks it's nothing more than Kurtz's madness taking over in his last moments of life. Perhaps he thinks it's Kurtz's damning judgment on the Interior, the corruption of the people around him, his own corruption, etc. We don't know for sure, but if you can find quotes in the book to back your thesis, you should be fine.