Given that Conrad was writing in the heyday of British colonialism, it is rather staggering the kind of criticism that he levels at colonialism in general in this powerful novella. Conrad uses Marlow as his voice to gently critique both colonialism, and through this, civilisation, as the reader is forced to ask themselves very carefully if any nation that engages in such terrible acts of oppression and random violence can be actually considered "civilised." Note the following two quotes, both taken from separate sections in the novel, and consider how they critique colonialism:
They were conquerors, and for that you want only brute force--nothing to boast of, when you have it, since your strength is just an accident arising from the weakness of others.
The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it much.
For so many nations at the time, colonialism had a massive mythology supporting it. As Marlow's aunt suggests, it was thought to be about "weaning the ignorant millions" from their savage ways. It was thought to be an act of kindness, of Christian goodness. Marlow, in both of these quotes, cuts through such ideas by stating colonialism only involves "brute force" and is based only on taking land away from those who are slightly different to ourselves. He exposes the underbelly of the colonial project.