What is Kurtz able to do at death's door that Marlow is not?

"Kurtz was a remarkable man," Marlow says, because he "had something to say". Marlow didn't have anything to say. Perhaps this was what saved Marlow from insanity. Still Kurtz was able lo look into the whole mess of human nature and speak the truest words that Marlow could not, “The horror! The horror!"