Questions about Heart of Darkness by Joseph Conrad?

I don't understand what is so significant about Kurtz. Is he actually a good or bad character? I thought he was good but the fact that Marlow lied to Kurtz's Intended (fiance) about what Kurtz said when he died for the reason that it was too unpleasant for her to know makes me think that he was bad. I'm guessing that he's a bad character for the reason that he forced labor upon the natives for getting ivory but then why would they attack Marlow and his ship to prevent them from taking Kurtz away? I don't seem to understand the basic idea of the book so please let me know what makes him a good/bad character and why.