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.