It is hard to say that the monster represents one side of human nature, because the monster is a product of his surroundings and of the treatment from his creator, Victor Frankenstein. The monster is at first very nurturing and caring. He wants to belong and tries to take care of the Delacy family. This is even after the fact that Victor has been repulsed by him and alienates him. The monster only seeks acceptance. He wants victor to create a mate for him and promises to leave the world of normal man. Victor assumes responsibility for eradicating what he perceives as a mistake when really he should assume responsibility for teaching and training his "son" to fit into normal society.
The other side of human nature is seen when the monster is refused a mate, cast out of his "family" and treated poorly by virtually all normal men, save old man Delacy. True, he should not have murdered Victor's little brother, but the dark side of human vengeance is irrational and impulsive. The monster wanted to take away something that mattered to Victor, as much as he lost something at being alienated.