The creature also represents the parent/child relationship and the abandonment issue. Shelley, whose own mother died just days after Mary was born, felt this keen sense of abandonment her entire life. She had to resort to stories her father told and the writings her mother left behind in order to get to know the person her mother was. Much like the creature, Shelley felt deserted and neglected...obviously through no fault of her mother's, but nonetheless, it had a profound impact on her life and comes out vividly clear within the themes of the novel.
Shelley entertains that the parent has an undeniable responsibility to his/her creation/child. This is a responsibility Victor did not take. By denying his "son" the unconditional love the child deserves as birthright, Victor has "played God" or father to "Adam/the creature" with terrible and terrifying results. This is Shelley's lesson to the world and to all those who would toy with becoming parents who are not yet sure if they want that kind of responsibility on their hands.
Shelley was speaking out against the "Playing of God" as she and many of her contemporaries viewed the outlandish scientific experiments which were being conducted during this time period.
The monster, the result of successful scientific experiment, represents the evils of science. Shelley was writing in time when science was taking new leaps and bounds, concerning many people who felt that nature was being taken for granted, and who sensed that the power science was giving humans was actually dangerous. In this novel, she has the protagonist create the monster to demonstrate that: 1) Science should not take responsibility for tasks that are a part of nature (such as creation), because 2) Nature can not be controlled.
Victor Frankenstein creates his monster, abandons him, and assumes all is done. But he can not control what he has created. The monster thinks and feels and takes action. He, in fact, ends up controlling Victor's life. This is a strong message that if humans try to use science to control nature, it will actually be the science who controls the human.