The monster killed family, friends, and important people in Victor's life after feeling betrayed. He became to fall ill after all of his tragedies. I believe the main reason of his illness is that he feels so guilty that no one knows about his creation and that he created something that murdered people , created crimes and destruction. I believe illness brings out the end of the story. After losing the monster in the Ice he falls ill. This instance creates a different type of illness because of the obvious fact that Frankenstein does not recover, but ends up dying instead. For Victor, death is the only escape, allowing him to finally stop fighting the illnesses that always brought him nothing but misery and that he did everything he could to avoid fighting his illnesses. The fact that he found this escape of illnesses on the boat with Walton is no coincidence, however.
The illness lets him escape from any responsibility he has and also to talk about his guilt. He always regains his health, of course, because he has obligations to the people around him to appear normal and also to himself to stop the monster. Eventually, however, these obligations both end and he escapes into illness and never comes back, dying on Walton’s