In Frankenstein, why did Victor not create the mate he promised?
In chapter IX, the "monster" and Frankenstein discuss this. When the creature presses Frankenstein for an answer, he says, "You may render me the most miserable of men, but you shall never make me base in my own eyes. Shall I create another like yourself, whose joint wickedness might desolate the world...you may torture me, but I will never consent." Victor does vacillate, however, thinking he owes the creature, and that a mate will enable him to be moral, for he will have someone like himself and in that way be less of an outsider. Then, in Chapter III of the next book, after he is almost finished creating the mate, he destroys it because he sees the monster looking at him, and he sees on his face "the utmost extent of malice and treachery." He cannot create another, no matter what the consequences.
Victor decides not to create a mate for the creature because he realizes the consequences behind creating a new life and not taking full responsibility for this creation. He has played God by bringing this creature to life but has failed miserably to nurture and educate this new being. His abandonment of the creature results in its transformation into a monster, which becomes capable of hate and murder because of feeling alone and unloved in the world. Allowing himself to make another similar creature or "mate" to "please" his original creation will not remedy all the harm he has caused nor will it guarantee that his original creature will now have the love and security it craves. This is not to even mention the disastrous consequences if the creatures procreate!
Although the monster has promised to go far from where people live with his mate when she is made, Victor is afraid that something may go wrong. He has no guarantee that the mate will be attracted to the monster, and if she rejects him, his wrath will be terrible. If she turns out to be ruthless like him, there will then be two fiendish beings loose to contend with. Also, even if all goes as planned and the two do retire to an area where no one lives, Victor considers that they may procreate, leaving more of their kind to potentially wreak havoc on the world.