How does Frankenstein and creature of Mary Shelley's Frankenstein represent two opposites sides of humankind?
Victor represents all the superficial, mundane, and inherently bad aspects of human nature: Ambition, Selfishness, Self Absorption, Vanity, Self-Gain, and a God complex. This is evident in how it was precisely this combination of factors what came together to create the monster and, ultimately, destroy his own life.
The creature represents an introspective look at the self, and the nature of others. It is a more feeble and needy side of humanity which longs for connection, meaning, and hope. It is the core of all the feelings that humans collectively share, such as love, hate, fear, and misery.
What is ironic is that, although Victor was the genius of the two, he was also irrational enough to ruin himself. The creature, although being a "creation of dead body parts" was able to educate himself, and build enough humanity to even desire a partner. Both ended up in the same state. Yet, each is the antithesis of the other.
Frankenstein's creature represents both good and evil in mankind. The creature is man at his basic animal nature when he kills Elizabeth and Cerval. Because of his murder he has indirectly also caused Justine's death. He is raw and savage initially, but he changes.
The creature teaches himself to read. He finds delight in books. He observes people from a distance and longs to be a part of them. He desires love, touch affection, and companionship. He develops good speech and talks eloquently to the captain in the last chapter after Victor dies. He pleaads his case by sharing his own torments caused by man's rejection of him.