The man he replaced was killed by the son of a native cheif. The squabble took place over a disagreement over some chickens, two black hens to be precise. See the excerpt below:

Fresleven--that was the fellow's name, a Dane--thought himself wronged somehow in the bargain, so he went ashore and started to hammer the chief of the village with a stick. Oh, it didn't surprise me in the least to hear this, and at the same time to be told that Fresleven was the gentlest, quietest creature that ever walked on two legs. No doubt he was; but he had been a couple of years already out there engaged in the noble cause, you know, and he probably felt the need at last of asserting his self-respect in some way.

Fresleven was beating the chief to death with a stick when the cheif's son ran him through with a spear. Marlow eventually finds his predecessor's bones, still in the same spot with the grass growing up through the ribs.

Fresleven's story is but one example of how the darkenss pervades (or is revealed in) the hearts of the men who go to Aftrica. He was there for the same noble cause (to bring the light of civilization/Christianity to the dark) as Marlow and Kurtz, but he, despite his "gentleness," was overcome with darkness.

The excerpt I shared is from section 1 of the novella. Enotes offers the entire etext at the link below. I have also included the enotes page that contains summary and analyis for each section.