Kurtz is initially sent to the Congo to bring back ivory for the Company. This was standard procedure in that era; Kurtz was a "boss" overseeing white men and native slaves, and his output of ivory dictated how much support (difficult and expensive to send) he received in turn from the Company. However, Kurtz discovers that he values the power he holds over his men more than his rewards for service, and is overpowered by that lust.

But his soul was mad. Being alone in the wilderness, it had looked within itself, and, by heavens! I tell you, it had gone mad. I had -- for my sins, I suppose -- to go through the ordeal of looking into it myself. No eloquence could have been so withering to one's belief in mankind as his final burst of sincerity.
(Conrad, Heart of Darkness, gutenberg.org)

Kurtz is obsessed with his own power. He can control men through his voice and his physical prowess, and has set himself up as almost a god among his men and the natives. Only his sickness causes him to lose that power. In the end, he did not care about the Company or even truly about his Intended, but instead cared only for the feeling that wielding that power created. While he may have recovered his sanity in time, his ill-health prevented that, and in the end Kurtz's journey was destroyed partially by his own obsession.