Understanding what Roderick represents in Poe's "The Fall of the House of Usher" is a subjective exercise at best, based upon perceptions of readers and literary critics, for no one has the ability to ask questions of the author.
Depending upon one's opinions, this could be seen as the story of a man haunted by the ghost of his dead twin—for at the story's beginning, Madeline appears in the room, ghost-like. Other possibilities could be found in readings of the story. I have selected the one below.
From my reading, I agree with others that perceive that Poe's story is a tale of incestuous twins following in the footsteps of ancestors that practiced the same horrific behavior. While Poe does not address this as a religious moral tale, morality plays a part in the story. This behavior is unnatural to the extent that children born of such a union are often physically and mentally damaged. It cannot be known what, if any, familiarity Poe had with the physiological ramifications, but it is safe to assume that he was aware of the mental ramifications for children born of an incestuous relationship.
[Poe] seems to be suggesting that, despite the incestuously twisted and mentally deranged life of the Ushers, there exists an unwritten but operative universal morality that is ultimately as inescapable as the hereditary forces that determine a person’s life.
In that Poe's story alludes to forces at work beyond what is seen and generally understood in the physical world, the reader can also assume that some supernatural force is present. ["Supernatural" refers to anything that goes beyond the boundaries of the natural world.] It would seem that this force may be working to exact justice on the Usher line.
Roderick is certainly a victim and a perpetrator. Regardless of the unnatural actions that created him and his sister Madeline, he has continued the practice. However, while the reader might want to pronounce Roderick to be evil personified, he is something of a sympathetic character because his mental state is something for which he cannot be held responsible. Mentally and physically he is broken beyond measure. After the narrator has had a brief opportunity to visit with and study this childhood friend (that he realizes he no longer knows), the reader may well note a similarity between the facade of the house and that of Roderick.
The narrator described on the edifice...
...a barely perceptible fissure, which, extending from the roof of the building in front, made its way down the wall in a zigzag direction, until it became lost in the sullen waters of the tarn.
Upon examining the physical characteristics of Usher himself, the reader can easily see that the same kind of fissure, or crack, running through Roderick as well, hinting not only at instability, but also foreshadowing an imminent and disastrous event caused by the instability. Interestingly, at one point in the story, Roderick describes the house as having a dreadful affect on him, as if it were alive.
Roderick's appearance (the narrator notes) has radically changed—mirroring the brokenness of his ancestral home. He has a wild look about him and over-bright eyes that startle and awe the narrator. Roderick is hyperactive: experiencing "nervous agitation." At other times he seems to exhibit the characteristics of an opium user. In truth, Roderick's behaviors are so diverse and ever-changing that they seem to operate in direct contradiction to each other.
Roderick tries to describe his malady:
It was, he said, a constitutional and a family evil, and one for which he despaired to find a remedy...
As the story progresses the reader becomes more aware of the strange relationship between brother and sister. The reader may be reminded once more of the "family evil" of which Roderick spoke.
Though Poe never outwardly accuses the brother and sister, too much information is present to ignore the veiled references. This is a family in which decent of the family line has followed the male side of the family, whereas the last name (regardless of the heir's gender) has always remained "Usher."
...the entire family lay in the direct line of descent...
This also supports suspicions that an incestuous relationship exists between brother and sister.
Roderick is physically and mentally unwell. Madeline is also ill, suffering from an unknown illness. Ultimately she "dies," and her brother (who has no other family in the world) places her body, with the narrator's help, in a temporary coffin, lid screwed in place, behind the enormous and solid doors of the family vault.
It is only after Madeline appears again, somehow having escaped the confines of her entombment, that the narrator realizes she was not dead at all, though Roderick knew. This shows the extent of Roderick's self-hatred, aware of the implications of not only his actions, but those of his ancestors as well.
...it is precisely Roderick’s morality that causes the internal conflict he suffers, between his inherited traits and his moral revulsion over them, and it is his morality that prompts him to leave Madeline in the vault even after he discovers that she is still alive.
Ending the age-old cycle of his family's immoral actions becomes Roderick's tragic act of attempted murder and self-sacrifice:
Roderick’s sense of right and wrong has transcended concerns for what is good for the Ushers and their perpetuation, and becomes a greater, higher concern for the future of the human race.
So, with all of this in mind one could argue that Roderick represents several things. His desire to put an end to the Usher family paints him as an agent of retribution. At the same time, his hatred of what he is and does—dictated by generations before him—may present Roderick as a hand delivering punishment. Finally, Roderick's self-hatred may be what motivates him to take steps to destroy the last of his family's line: including not only Madeline, but himself as well. In this way, he is representative of justice—and in that he does not expect that he shall be spared, regardless of the circumstances that have brought him to this place in the world, his self-sacrifice shows that his justice is even-handed.
In the end, Roderick successfully helps to orchestrate the "fall" of the line of Ushers. Strangely, the house mirrors his desire by falling in upon itself and obliterating the family's history, sins, name, descendants and—potentially—all memory of its existence. Perhaps this is the work of the "inescapable...universal morality" that Poe seems to describe within the story.