In what ways are Marlow and Kurtz considered to be "morally ambiguous" characters, and how does this illuminate the overall meaning of the story?

Marlow and Kurtz reveal a dichotomy in their personalities as they both are depicted as having positive and negative character traits and beliefs, which helps illustrate and support Conrad's view of humanity's inherently wicked nature.
While Marlow is portrayed as a relatively self-conscious man who detests the inefficient, abusive nature of the company and its employees, he is nonetheless fascinated by Kurtz and shares inherent personality traits with the enigmatic character. Marlow is obsessed with traveling deep into the African continent and ends up admiring Kurtz, despite witnessing his wicked nature. Marlow continues his journey even after witnessing the negative effects of imperialism each time he stops at one of the company's stations. Marlow also lies to Kurtz's "intended" and discovers that there is darkness in every human heart. Marlow's questionable decision to continue journeying deep into the Congolese jungle, his admiration for Kurtz, and his dishonest response to Kurtz's intended portray him as a morally ambiguous character.
Likewise, Kurtz is also a morally-ambiguous character, despite his obvious flaws and maniacal nature. Kurtz initially entered the Congolese jungle in hopes of exposing civilization to "savages" and creating a structured, organized society deep in the African continent. Unfortunately, Kurtz becomes a victim of his own inherent primitive nature once he distances himself from civilization. Kurtz's soul becomes corrupted, and he begins ruling as a tyrant and god over the natives. His inherent wickedness flourishes in the uncivilized environment void of rules, laws, and regulations. Kurtz can be considered a morally ambiguous character because he begins his journey into the Congolese jungle with positive intentions only to become a victim of his own inherently wicked nature.
Conrad's depiction of Marlow and Kurtz as both being morally ambiguous people illustrates his negative views of humanity. Conrad's novella portrays humans as inherently wicked beings and challenges the reader's perception of civilization and humanity.