In Mary Shelley's Frankenstein, by the time Victor comes to animate his first creation, he is already in a highly nervous state, filled with premonitions of disaster. These, however, are far more intense in his second creation, as he has now seen the horrifying effects of the experiment. In physical terms, Victor appears to follow a similar process both times, but his motivations are quite different. There is no ambition or scientific curiosity in his second creation. He is motivated principally by fear of the creature, who has threatened Victor with the death of his friends and family if he does not create a female companion for him.
As he works on his second creation, Victor thinks of all the disasters that could ensue. The female creature might turn out to be more malevolent that the male one which is currently blackmailing Victor. She may not want to become the mate of the first creature, and this rejection would only increase the first creature's fury. If she does accept him, and they have children, they could create a new species of hideous and destructive monsters. Victor even imagines that such creatures might be responsible for the extinction of humanity:
Had I right, for my own benefit, to inflict this curse upon everlasting generations? I had before been moved by the sophisms of the being I had created; I had been struck senseless by his fiendish threats; but now, for the first time, the wickedness of my promise burst upon me; I shuddered to think that future ages might curse me as their pest, whose selfishness had not hesitated to buy its own peace at the price, perhaps, of the existence of the whole human race.
Victor therefore concludes that, despite his promises to the creature he has already created, it is his moral duty to destroy his second creation and not to meddle in such matters again.