Ahhhh, you see, Marlowe is trying to do what he tried to do before.
Being a savage fighter (savage used as an adjective to the noun "fighter") does NOT make you a "savage" (used as a noun) or even "uncivilized" (used as an adjective).
To put it in other terms, someone who is civilized (lives in a town, farms & raises food animals) can be a viscious (savage) fighter when threatened. This does not make him a "savage".
savage (noun)
savage (verb)
savage (adjective)
savagely (adverb)
Semantics.
A savage can fight savagely.
A civilized man can fight savagely.
What Marlowe is attempting to do is to "prove" that they were savage fighters (they were)......
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So they were savages.
And uncivilized (despite having towns hundreds of years before the white man landed).
They were a different culture (actually, many different cultures).
Now, since we defeated them, that proves we are superior.
Just as cancer cells will destroy other cells and "prove" they are "superior".
Being able to destroy someone else only proves that they were weaker than you. Not that you are morally superior or culturally superior or intellectually superior or anything else. Just that they were weaker than you.
A simple test of this is to go down to your local biker bar and spit on the biggest, toughest, meanest, DUMBEST biker you can find. Preferably one who has low marals and lacks cultural refinement.
Then claim that you will defeat him because of your superior moral structure or culture.