What is the point of Marlow's disquisition on the various kinds of devils he's seen? (outer station)

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.