As he departs for England with his friend Henry, hoping that the presence of Henry will prevent the intrusion of the dreaded creature, Victor is anxious about his "friends" who are not aware of the existence of "their enemy." They would, thus, be vulnerable to the creature's attacks if he becomes "exasperated" by Victor's departure.
Victor's use of the word "friends" includes his family and Elizabeth. Victor reasons that the creature will follow him because this monster has promised to pursue him no matter where he goes: "But he had promised to follow me wherever I might go; and would he not accompany me to England?"
Yet, while Victor surmises that the creature may wish to make certain a mate is made for him and follow his creator, there is also the possibility that the creature might remain behind and prey on those Victor loves. Since his loved ones are unaware of the existence of this enemy, and are unprotected from attacks of such a creature, Victor is greatly disquieted, and he feels much uneasiness about his family's safety. Victor describes himself as "agonised" with the idea of the possibility of harm to those loved ones that are yet part of his family after the recent tragedies.
Frankenstein’s greatest fear as he left for England was that the monster would harm his remaining family members. The monster had already killed his younger brother, William, and framed Justine by planting William’s locket on her. Failing to prove her innocence, Justine was consequently executed, leaving Frankenstein with an intense feeling of guilt. After the monster narrated his sad tale of human rejection and requested that Frankenstein create a loving companion for him, Frankenstein felt obligated to do so on condition that the monster and his companion move to a remote area where they would never hurt human beings. Although the thought of two evil monsters and the destruction they would cause to human beings repulsed Frankenstein, he dreaded the consequences in the event that the monster failed to honor his promise.
In chapter eighteen of Mary Shelley's Frankenstein, Victor decides to go to England to create the mate the creature has desired he make.
I expressed a wish to visit England.
Victor has two different fears which accompany his decision to go to England. First, Victor's health has returned. After years of illness, Victor is finally feeling better (mentally and physically). He fears that leaving Geneva will force his illness to return.
Second, Victor fears the task at hand. He must create a mate for the creature. If Victor fails to create a mate for the creature, the creature has promised to be with Victor on his wedding night (most assuredly murdering another of Victor's family members--namely Elizabeth).