Marlow reports that it is the manager's boy who announces the death of Kurtz:
Suddenly the manager's boy put his insolent black head in the doorway, and said in a tone of scathing contempt:
"Mistah Kurtz—he dead."
Marlow had until recently been with Kurtz, and he reported Kurtz's last words as "The horror! The horror!" before blowing out the candle and leaving his cabin.
Because of this, we tend to remember Marlow as witnessing Kurtz's death. However, at the point of death, Marlow is eating with other people in the mess room, sitting across from the manager. This means that some time passes between Kurtz saying "The horror!" and the report of his demise. It is possible, in fact, that these were not Kurtz's last words, pointing to gaps in the narration, although Marlow implies that they were.
Marlow stays in the mess hall eating while the others rush to see the dead Kurtz. Marlow says he does so because there is a light on in the mess room and it is so terribly dark outside, symbolizing the evil and uncertainty he does not want at this moment to confront. He says he believes the others considered him "brutally callous" for staying at dinner in the wake of the death, not realizing it is too much feeling, not too little, that keeps him away.
After this, Marlow suggests that he caught the fever and almost died, but notes that it was his "destiny" to live on and have to tell this tale.