In Mary Shelley's Frankenstein, Victor is persuaded by the creature to creature a mate for him. At first, Victor refuses, but the monster reminds Victor that he is lonely and another being like him would not turn him away.
I am alone, and miserable; man will not associate with me; but one as deformed and horrible as myself would not deny herself to me. My companion must be of the same species, and have the same defects. This being you must create.
Victor still refuses, saying that do to the same thing again would make him "base" (low) in his own eyes. He does not want to take the risk of creating a pair of murdering monsters: at the moment, there is only one. However, the creature threatens him:
I will revenge my injuries: if I cannot inspire love, I will cause fear; and chiefly towards you my arch-enemy...I will work at your destruction, nor finish until I desolate your heart, so that you shall curse the hour of your birth.”
Victor finally gives in. He travels to England to try to learn as much as he can to help him with his new task, and then returns to begin. As Victor starts the process on a small Scottish island, he continues to battle with himself: should he do as the creature asks? His work disgusts him so much, that finally one night he tears the this new "being" to pieces. The monster, outside the window, sees Victor's actions and is insane with fury.
The pros of Victor destroying the second creature is that there will only be one monster roaming the earth. Two creatures could harm a great many people; if they mated, several of them could descend in anger on mankind and kill many individuals. In not creating the mate, Victor will not repeat his mistake of playing God the first time.
The cons are powerful—the monster has threatened Victor and his family if Victor does not fulfill his promise. Victor loves his family very much. He already knows what the creature is capable of: he has killed Victor's youngest brother, William, and caused the death of an innocent Justine.
Once Victor makes his decision, it is not long before the creature wreaks his revenge: he murders Clerval, and ultimately murders Elizabeth on their wedding night. Victor's father's heart gives out from the agony of losing Elizabeth, and he also dies.
Once Victor creates the monster, there is no going back, and no easy solution.