Conrad's works had a diverse variety of subjects and settings owing to the direct consequence of his life as an explorer. According to G. Jean-Aubrey, Conrad's work and life got amalgamated to a great extent and it is his journey to the Congo in 1890 which gave birth to the three major works including Heart of Darkness. This answer will try to examine the issues of imperialism, civilization and being as accommodated by Marlow's journey in Heart of Darkness. A childhood fascination, his aunt's influence and a lucky opening is what sets Marlow on the journey to Africa.
His first stop on the Congo basin is the Outer station, where he is welcomed by the image of a railway truck lying upside down like some dead carcass. This is the foreshadowing of what is to come, a scene of decay and chaos caused by the imperialist project. There is mindless blasting of hills to build a railway line where there is no obstruction, the imported drainage pipes are dumped causing them to break and hence rendering them useless and the inhuman treatment meted out to the natives where they are tied down like animals with iron collars and ropes which presents a very wicked picture of colonization.
This madness is further established by the fact that the object of procurement, ivory, is of no intrinsic value to the colonizers; it is just the fetish that leads them to unleash the eternal "gloom" on natives. The only European that Marlow encounters at the Outer station is the Company's chief accountant, dressed in impeccable white, surprisingly untouched by the sad "darkness" of the place. His representation as a fop, perhaps, is symbolic of the fact that in order to survive there one needs a certain degree of obsession and a greater one of indifference.
The white man's burden falls on his shoulder and the only feat he achieves, ironically, is teaching the black woman how to starch his clothes. The juxtaposition of his white clothes with the withering black skins of the natives left to die exposes the very sham of civilization that imperialism seeks to provide. This leads to the second matter, of civilization and society in Heart of Darkness. A civilization is essentially characterised by rules, laws, development and culture. The picture provided to the readers at every station is very much the opposite of all that is stated above.
Chaos is the keyword and power is abused at every level. Somewhere a poor black man is beaten up just to pin the blame of fire which burned down the grass shed on someone and at another place, the severed heads of so called "rebels" are put on the top of the fence posts like ornament balls. There is complete lack of law (or power is the only law), even of pity or other human emotions for the natives which makes the so called civilized people akin to rather worse than the brutes.
Achebe uses this portrayal of Africa as an anti-thesis to civilization, the treatment of natives as mere "limbs" and "glistening white eyeballs" without a voice or a personality against Conrad to say that he's a "thoroughgoing racist" but the point he misses here is that Conrad does not make any attempt to redeem the Western civilization. The racist language used is apt to be heard from the mouth of somebody like Marlow, who is an Englishman and still exploring the darkness. Edward Garnett identifies the subversive nature of the text and Conrad's treatment of "the other" in works like Lord Jim proves that the motivating factor is not racism.
He lived in the Age of Doubt where people like Charles Darwin, Matthew Arnold and Sigmund Freud questioned the various beliefs of society and Heart of Darkness is perhaps his experiment with the same. Conrad seems to challenge the strength of societal construct which if devoid of moral and ethical implications, crumbles down. He proves the weakness of the structure itself irrespective of the African backdrop and this motif of degeneration continues from the society to an individual in the novella. The journey fits almost perfectly in the Freudian model of the Structure of the Unconscious to explain this degeneration.
Marlow reaches the Outer station with certain sanity and fixed notions of right and wrong. Hence, this stage can be associated with super-ego, where almost everything he notices shocks him because it is not in keeping with the standards of the society. By the time he reaches the Central station, these notions become a little flexible. Besides that, he realizes the importance of the journey, his importance to the journey, and takes decisions like throwing the helmsman's body into the river (which may have been absurd for him earlier) revealing a balance between the superego and the id also known as ego.
Id is manifested in the novella through Kurtz in the Inner station given to his pleasures and his belief that "Everything belonged to him". Marlow might have been another Kurtz had he not restrained himself from taking the plunge. It can also be seen the other way round where Marlow is what Kurtz might have been before his degeneration into Id.
The final nail in the coffin is the failure of the journey with Kurtz's death since it was the very heart of the quest and the last words uttered by him "The horror! The horror! are symbolic of the catastrophic point where even words lose their meaning to Id, turning into nothingness. A larger view of the same concept can be the sequential erosion of Europe through the journey where the weak shackles of civilization slowly break away and are replaced by the very instincts to survive, leading one to the Inner station where the end is madness and total submission to Id; where everything else is rendered unimportant. So, in my view, it is the journey, which justifies the end (rather than being the other way round) and ties all the strands of the narrative together to hold Heart of Darkness together.