The creature uses two techniques: fear and guilt.
Victor already realizes that the creature that he has created is responsible for the murder of his younger brother and, in turn, the execution of one of his closest friends. The creature guarantees Victor that he will allow no happiness and much more death and destruction if Victor does not create a mate.
Also, the monster appeals to Victor's sense of guilt. He tells Victor about how horrendous his 'life' has been as he has wandered from place to place in search of love only to be rejected at every turn.
The creature appealed to Victor's guilt and fear. He tells Victor that because of the way he was created, he is the one creature in the world that is totally alone. All other creatures can have a mate to share their life with; he has no one. That appeals to Victor's guilt. The appeal to fear is more direct. The creature tells Victor that the only way to stop his killing spree, the only way to guarantee that Victor will no longer be a murderer.by.association, will be to create him a mate. He promises that he will go off with her, never to be seen again, never to kill again.
Victor is consumed by fear and guilt, and he is willing to do anything to "atone" for his son's/creature's action is at first willing to go along with this. In the process, however, he changes his mind and destroys the "mate." It's interesting to discuss why he did this. He says that he didn't want to create a race of "creatures," but how difficult would it have been to create a sterile mate? Probably no more difficult than making a functioning creature ... but, then, it would be a totally different story.