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.