In Frankenstein, why does Victor proceed with his plans to marry Elizabeth?...despite the monster’s earlier threat to visit him on his wedding night?
After receiving Elizabeth's letter in which she asks him "Do you not love another?" because Victor no longer shows an interest in marrying her, Victor's memory of what the creature has told him is revived --"I shall be with you on your wedding-night." Victor believes, of course, that his "daemon" will destroy him and prevent him from what little happiness he can attain by having Elizabeth's love to console him for all his losses. However, Victor resolves, "Well, be it so," deciding to marry Elizabeth and have what little joy he can before his inevitable death. At least, with his death Victor can finally achieve some peace.
In addition, Victor feels that if he marries Elizabeth, he can end her misery and give her some happiness, if only for a brief time: "Yet I would die to make her happy." Besides, Victor reasons, if the creature believes that he has postponed his wedding out of fear, the "monster" will merely devise another plan that is even more dreadful, for Victor's torture and destruction. So, Victor resolves to agree to his union with Elizabeth in order to bring happiness to her as well as to his father.
Here again Victor engages in rationalizing his situation and his future actions. While he deludes himself into thinking that he wishes to make his father and Elizabeth happy, Victor chiefly deceives himself by attributing all the evil machinations to the creature.