Heart of Darkness-symbol

When Marlow is on the steamer, he encounters endless amounts of fog, fog that entraps the men and the steamer. Marlow and the crewmen are surrounded by the fog, unable to make out anything or decipher friend from foe. The fog represented Marlow's mental state, and his inability to make a decisive decision regarding Kurtz, as well as, his own essence as a person.
The fog allowed him to make judgments about situations, however, it was too blinding to reveal the right answer. The path to take was unclear, similar to how he was unable to determine if he was guiding the steamer into open water or danger. Fog generally symbolizes confusion, and Marlow falls victim to this. The savages attacking represents Marlow's internal struggle for conflicting thoughts on Kurtz and his acceptance of his internal evil. Not only is he battling with right and wrong, he is fighting for life and death. Subconsciously, Marlow knows the decision he makes about Kurtz will determine his existence as an individual, and the confusion or fog Marlow encounters is also a brief moment of clarity because he knows the severity of his choice.
Since Marlow's arrival he has been confused and unsure - dealing from the treatment of the natives, cannibals and pilgrims - what originally seemed wrong, was now being justified as right; like his original perception of Kurtz. He believed him to be this awe-inspiring, self-creating legend, when in truth, he was insecure, insane, and immoral. The fog created was almost self-imposed because of Marlow's instability as a person. Had he been firm in his own beliefs and convictions on right and wrong he would not have faced such a conflicting situation with Kurtz, but because of this, he was able to become stronger and better for it.