The monster begins its life with a warm, open heart. But after it is abandoned and mistreated first by Victor and then by the De Lacey family, the monster turns to revenge. The monster's actions are understandable: it has been hurt by the unfair rejection of a humanity that cannot see past its own prejudices, and in turn wants to hurt those who hurt it. As the monster says when Felix attacks it and flees with the rest of the De Lacey family, "...feelings of revenge and hatred filled my bosom...[and] I bent my mind towards injury and death." But in taking revenge, two things happen to the monster. First, it ensures that it will never be accepted in human society. Second, because by taking revenge the monster eliminates any hope of ever joining human society, which is what it really wants, revenge becomes the only thing it has. As the monster puts it, revenge became "dearer than light or food."
Revenge does not just consume the monster, however. It also consumes Victor, the victim of the monster's revenge. After the monster murders Victor's relatives, Victor vows a "great and signal revenge on [the monster's] cursed head." In a sense then, the very human desire for revenge transforms both Victor and the monster into true monsters that have no feelings or desires beyond destroying their foe.