Answer:
If Marlow would have chosen the path chosen by all, he would have been a plain character for the story.
Step-by-step explanation:
People's personality and the factors which build each personality drive from one extreme to the other. You have to consider that Marlow is a character that acts as a leading figure through the story to show you two extremes. One is the European morality that was born from the current economical process. And the other is the savagery that natives were used to have. Now, Marlow travels from one extreme to the other demonstrating that the actual heart of darkness is the European morality that was leading institutions into bankruptcy and the savagery he found out in the Congo was just that. People who acted the way they were taught to survive. Even though it is possible to distinguish that from the plot. The main idea is that his choice makes him grow as a human.