Throughout the text, it is implicated that Kurtz has had mental problems in the past, but that he was a very powerful leader until he ventured too deeply into the jungle. The manager decries Kurtz's methods, but enjoys the great amount of ivory that Kurtz delivers. Everyone who has met Kurtz is affected by his intensity, but it seems that his mental breakdown comes only after he has lived in the jungle for a time. Marlow seems to believe that Kurtz ventured too far into the jungle -- not the physical, literal jungle, but the dark areas of the mind that are usually covered over by civilization.
Since I had peeped over the edge myself, I understand better the meaning of his stare, that could not see the flame of the candle, but was wide enough to embrace the whole universe, piercing enough to penetrate all the hearts that beat in the darkness. He had summed up -- he had judged.
(Conrad, Heart of Darkness, gutenberg.org)
In this interpretation, Kurtz would have been a very intense but sane man of varying morality; he is not necessarily evil to start, but his experiences in the jungle have warped and erased his moral code. While dying, Kurtz covers both his delusions of grandeur and his terror at his impending death; he still believes that he is destined for true greatness, despite his actions, but at the same time he is acutely aware that death means he will cease to exist. Also, Kurtz's Intended, the woman he left behind, believed him to be sane when they parted ways.