Marlow makes several references to the “red-eyed, devils” and “flabby, weak-eyed devils.” Who is he referring to when using this metaphor? Why would he call them by those terms?

I think Marlow distinguishes between the “devils” of violence, greed, and desire in Europe with the “flabby, pretending, weak-eyed devil of a rapacious and pitiless folly" that he finds in Africa. The devils in Africa are also white European men but far more insidious and evil then any he knew in Europe. They are metaphors for evil, there only to infect the country and its population.