Victor is the greatest example of free will at work. As a representation of science, he is a living symbol of each human's control over his/her own life, and his pursuit of knowledge at any cost leads to the creation of the monster. His choice to run away, rather than face what he has done, is a direct cause of the deaths of his loved ones. In fact, before Justine's execution, he makes the choice not to reveal his secret, which could have saved her life. These losses add to his psychological burden, which results in his eventual mental breakdown and death.
The monster too, as a rational being, exists as an example of the consequences of free will. Although he had no choice in his creation, no one does. His rejection by Victor and society influences his decision to seek a life of revenge. However, that is ultimately his choice, and his loss of a father (in the figure of Victor), coupled with his loss of a mate (in the figure Victor destroyed) are direct results of that choice.