While there are many labels that can be applied to Kurtz and what was done to different areas of the world under colonization, there was a sense of adventure present. Marlow has to find Kurtz through progressing through the Congo in order to reach his goal. The journey to find him is both adventurous on a physical level, as traveling down the Congo is arduous in its own right, and on an emotional level, whereby the more Marlow learns of Kurtz, the greater the complexity in their relationship emerges. In terms of Kurtz himself, the things he did in the name of "civilization" as well as the reality that there is this darker side present is one where adventure was present. It might not have been good adventure, but it was adventurous nonetheless. There might even be an argument made that Kurtz's final words helped to spawn a great deal of intellectual adventure in terms of their meaning and implications.
The Heart of Darkness is actually the story of two men's adventures. Joseph Conrad works very hard in developing Marlowe as a narrator and a well traveled sailor as he goes into the African Congo but by the same token, he also develops Kurtz as a strong supporting character as we begin to understand his role as the chief of the stations where the African natural resource of ivory was trafficked. Marlowe's constant urge to find and talk with Kurtz as Kurtz becomes the epitome of the governing power in the Congo of Africa and all that that entailed including its ivory, its women, and Kurtz's eventual death. So, it's double adventure and one well to be enjoyed by the reader.