What is the impact of the monster's vow to return on Victor's wedding night?
Victor destroys the female creature he had promised to create in order to satisfy his first creature (who is miserable because he has no companion), prompting the creature to vow that Victor "'shall repent of the injuries [he] inflict[s]'" and that he will "'be with [Victor] on [his] wedding night.'" Most readers can immediately identify this statement as a danger to Victor's spouse rather than Victor himself: so far, the monster has only sought to deprive Victor of his loved ones, rather than his own life. Death would be too swift, and the creature wants Victor to feel the intense loneliness that he, himself, feels. Victor, with his typical arrogance, interprets these lines as a threat to himself and thus he takes no steps to protect his fiancee, Elizabeth. Had Victor been less self-centered, he might have realized that the creature would take away his chance at happiness with a partner, just as Victor has taken away the creature's chance.
Elizabeth, however, feels "a presentiment of evil" pervade her on their wedding day. Victor, instead of focusing on her feelings, focuses on his own, telling her,
"'Ah! if you knew what I have suffered, and what I may yet endure, you would endeavour to let me taste the quiet, and freedom from despair, that this one day at least permits me to enjoy.'"
Victor is so focused on his own feelings, his own concerns, that he fails to take Elizabeth's seriously or to consider her safety in regard to the pattern his creature has established. Thus, Shelley employs dramatic irony (when readers know more than the characters) to build tension until the moment the creature takes her life.