The letters close the “frame” in the novel. Walton’s version of the story is used to make Victor’s story more believable. Walton gives some validity to the story by mentioning that he sees Victor’s letters and the monster.
The first letter reinforces the theme that using knowledge for evil leads to disaster. Walton and Victor also talk of literature, probably Romantic books.
In the second letter, Walton has deep feelings about failure, sounding a depressed note on his failure to accomplish his goals. He also feels a deep sense of sorrow when he does find companionship, only to lose that companion in death.
In the letter dated September 5, Walton knows the limits of his personal and physical being, but Victor still wants to press on. Victor obviously has lost his mind, as no thinking person would risk their life for something like this unless it was really self-serving.
In the final letter, dated September 12, the monster alludes to Milton’s Paradise Lost by saying, “But it is even so, the fallen angel becomes a malignant devil. Yet even that enemy of God and man had friends and associates in his desolation; I am alone.” The monster knows that even the Devil had a host with him for aid and comfort. Being alone drove him to commit murders for revenge to torment his creator.
