Mary Shelley's Frankenstein is a novel about revenge. While many themes are present, the most prominent theme refers to the revenge both Victor and his Creature desire. Many times throughout the novel, both Victor and the Creature state their intent--to avenge the wrongdoings of the other.
Victor, after learning of William's death, promises that he will make the Creature pay. After the death of Justine, this promise becomes even more solid. Victor knows that the creature is responsible for the death of both William and Justine and swears he will make the creature pay.
The Creature warns Victor that if he does not make a mate for him that he will enact revenge upon both Victor and his family. Given the Creature has already murdered William, Victor cannot allow the threat to be ignored.
Over the course of the novel, many threats are made. That being said, it is the journey to the ends of the earth (by both Victor and the Creature) that Shelley's shows Victor's and the Creature's obsession with revenge. Victor, only upon his death, is able to stop his obsessive pursuit of the Creature. Likewise, Victor's death concludes the Creature's ability to enact revenge upon Victor. The fact that the plotting of revenge does not expire until Victor does proves that both are willing to go to the end of the earth in order to do as they promised.
Victor Frankenstein becomes obsessed with revenge after Justine, a servant who works at the Frankenstein family's house, is wrongly executed for murder. The creature placed a picture of Victor's mother in Justine's pocket to frame Justine for the murder of William, Victor's brother. Shelley shows that Victor is obsessed with revenge because Victor blames himself for Justine's death. Victor thinks to himself in Chapter 7:
"Did any one indeed exist, except I, the creator, who would believe, unless his senses convinced him, in the existence of the living monument of presumption and rash ignorance which I had let loose upon the world?"
Victor is particularly bent on revenge because the monster has destroyed people who were very dear to him—his brother William and the innocent servant, Justine. Victor is also bent on revenge because he feels incredibly guilty about the evil creature he has created. He says in Chapter 9, "When I reflected on his crimes and malice, my hatred and revenge burst all bounds of moderation."
The creature, for his part, detests his creator for making him an outcast among humans. The creature says to Victor, who created him, "Accursed creator! Why did you form a monster so hideous that even you turned from me in disgust?" The creature begs Victor to create a mate for him so that he, the monster, will not be alone. When Victor agrees and then reconsiders, believing that the monster's mate will also become destructive, the creature exacts revenge. He kills Victor's beloved friend, Henry Clerval, and Victor's wife, Elizabeth. Victor pursues the monster all the way to the Arctic to seek revenge and dies in the process. In the end, Victor and the creature are locked in a battle to the death in order to seek revenge on each other.