From the start, Marlow sets up savagery as a human condition. Early in the novel, he talks about what it must have been like for the Romans to first encounter the savage British thousands of years ago:
Sand-banks, marshes, forests, savages,—precious little to eat fit for a civilized man, nothing but Thames water to drink ... Land in a swamp, march through the woods, and in some inland post feel the savagery, the utter savagery, had closed round him—all that mysterious life of the wilderness that stirs in the forest, in the jungles, in the hearts of wild men.
The British, he implies, were (and are) no different from the Africans they are now encountering.
As Marlow enters into the jungle and encounters the manager desperate to find Kurtz, the physically hungry Marlow finds:
I was getting savage.
Savagery is a state of acting in way outside of the polite norms of European society. It involves the wild, shrieking grieving of the African natives that makes Marlow's hair stand on end. It involves barbarous treatment of other people: killing, maiming, hurting, enslaving.
In the end, Conrad's point is that the European nations, in their barbarous treatment of the Africans, are the true savages:
All Europe contributed to the making of Kurtz; and by and by I learned that, most appropriately, the International Society for the Suppression of Savage Customs had intrusted him with the making of a report, for its future guidance.
It is ironic that Kurtz's report for the suppression of savage customs becomes a blueprint for enacting savagery. Kurtz's behavior becomes the epitome of savagery, something so calculated, so outside of the norms of how he was raised, that it makes "pure, uncomplicated" savagery seem a relief. Marlow doesn't want to know what rituals Kurtz instituted to make the natives worship him. Marlow says it is enough to see:
those heads drying on the stakes under Mr. Kurtz’s windows. After all, that was only a savage sight, while I seemed at one bound to have been transported into some lightless region of subtle horrors, where pure, uncomplicated savagery was a positive relief, being something that had a right to exist—obviously—in the sunshine
Savagery cuts across time and space and lives in the hearts of all men. From this perspective, civilization becomes problematic. What it means to be civilized is to be what the European propaganda says: to benevolently help others; but as Marlow shows, this is a lie, a delusion. Marlow wants to put civilization into the hands of the women, by protecting them from the reality of savagery, but such a delusion is not civilization. In the end, civilization might be as little as sitting on a ship's deck and discussing humankind's failings.