While it is accepted that Kurtz is an evil man, he is also very complex; he has his own reasons for being in the jungle, but also performs his tasks as a gatherer of ivory better than anyone else. The manager who sends Marlow out to find Kurtz calls him:
"'He is an emissary of pity, and science, and progress, and devil knows what else."
Kurtz is able, through his great charisma and ability with language, to affect men and cause them to admire him. He is also capable of incredible brutality, and has used his control over the native peoples in the jungle to enforce his role as dictator through fear and murder. He claims "great plans" for the future, but doesn't quite seem to know what those plans are. Marlow describes how:
"Kurtz discoursed. A voice! a voice! It rang deep to the very last. It survived his strength to hide in the magnificent folds of eloquence the barren darkness of his heart."
(Conrad, Heart of Darkness, gutenberg.org)
This voice is Kurtz's greatest power; despite the madness visible in his deeds, Kurtz's words can convince men to follow him into the madness and then, perhaps, out again, but he never gets the chance to try this second. The Russian, who admires Kurtz, claims that one cannot converse with Kurtz but only listen; this is possibly how the Russian kept his sanity, because he did not comprehend Kurtz's words, but only listened to their sound.
In the end, Kurtz was a man who did not possess the depth of character necessary to survive seeing himself outside of societal constraints. Kurtz allowed the vacuum of the jungle to absorb his morality and his ethical ego, and what was left was entirely evil; he proved this through his actions. Kurtz may or may not have fully understood his own evil; his last words -- "The horror!" -- could be self-realization, or simply fear.