his predecessor, Fresleven, has died there while "engaged in the noble cause." what was the noble cause, and how did Fresleven die?

"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." The noble cause was "taming" the natives and stealing their treasures. Eventually the Chief's son stabbed Fresleven through the shoulder blades and killed him. Marlow will see plenty of brutality inflicted on the natives.

The noble cause was that of all the people in England; to civilise the Africans. This was a veneer to the darkness that they all constituted, they were exploiting the Africans rather for wealth and swindling resources from the African continent.