When Marlow finally makes it to Kurtz in the depths of the rainforest, he finds a disturbing site. The poles outside Kurtz' home are not merely decorative balls. They are human heads. 

The full extent of Kurtz's authority at the Inner Station is now revealed to Marlow. There are heads of ‘‘rebels’’ on stakes surrounding Kurtz's hut, and Marlow speaks of Kurtz presiding over ‘‘unspeakable’’ rituals.

Kurtz has either suffered or chosen a break with the morals of society. He has become a savage - much more savage than the figures in the novel who are given the moniker "savage". 

 ‘‘The horror! The horror!’’ With this utterance, Kurtz presumably realizes the depth to which his unbridled greed and brutality have brought him.

The severed heads on the poles outside his house are a graphic image of how far Kurtz has gone in his break from recognized moral codes. The limits of his behavior are determined only by his own imagination.