When Victor agrees to make the female companion for his monster, he does so for selfish and emotional reasons. He wants to stop the monster's killing spree and to protect his loved ones. However, when he has actually created the creature's "bride," he considers his actions from an unselfish, rational position.
The female creature, not yet alive, has not agreed to the conditions presented by the male creature. She may choose to seek out human companionship or go off on rogue, violent rampages of her own. Moreover, the couple might reproduce, ultimately creating a race of stronger and more adaptable giants. These giants could then eventually snuff out the humans.
Victor's decision to destroy the female is on a higher moral plane, and the reader undoubtably admires him for doing this. The reader might consider that he or she would not have been able to make such a sacrifice for the greater good. Some readers might feel sorry for the isolated and lonely creature and hope that Victor gives him some companionship, even though Victor does not seem willing to do so. Even though he created the problem in the first place, Victor makes a great sacrifice for all of humanity at a great personal loss for himself.