There was little respect for the local people, who were called savages. Europeans turned them into (chained) slaves, and often made them work under terrible circumstances. Europeans exploited the natives and their land, taking their products from Africa to Europe. In the case of Congo, ivory and rubber made trading companies lots of profits.

The story begins in London where five men on a boat talk stories. One (Mr. Marlow) tells the story of his search of a Mr. Kurtz, who is the manager of a Trading  Station in Congo. Mr. Kurtz was sent there by his Trading Company but since ivory wasn't coming out of the country anymore, Mr. Marlow was asked to go explore and bring him back to Europe. Mr. Marlow describes his adventures to Africa and on the Congo river to the 'heart of darkness' where he eventually finds Mr. Kurz. Turns out this Mr. Kurz, having been in the jungle for many years, let the environment and isolation get the best of him. He gave up all European civilization and 'turned dark''. He turned part savage, murdered anybody who bothered him and as a trophy would put their heads on a pole in front of his house (see fragment).  He had also fallen sick and ends up dying before Mr. Marlow was able to return him to Europe.  Mr. Kurtz represents the darkness in humans. When you are in a completely different world, without laws and order, would greed and isolation get the best of you? Would you do things you had never thought before possible because nobody is looking and you can get away with it? What kind of person would you turn into?  Well, Mr. Kurtz didn't end up well and ended up going mad. Mr. Kurt did whatever he pleased turning  the local population into his army killing anybody who came too close. 

The book reminds us that imperialism had many 'dark sides' such as slavery, exploitation, murder, betrayal, greed, etc. The disregard for life and suffering is a continual theme of the book.  An important message of the book is to make people aware of the fact that when there are no laws in place, or laws are not being followed because there's nobody present to police any wrong doing, people may show their worst behavior (dark side). This is what happened to Mr. Kurtz. Because he was living in an isolated wilderness with nobody to monitor his behavior, he  could get away with robbery, murder and even decapitated heads in front of his house. He showed his worst behavior because there was no one there to hold him accountable.