[Y]et one of the first results of those sympathies for which the daemon thirsted would be children, and a race of devils would be propagated upon the earth.
In Chapter 20, one of his concerns about creating a female companion for the monster is that they would reproduce and start a new race of beings. Read more about this quote in Quotes by Theme: Parental Responsibility.
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.
In Chapter 20, Victor realizes what he promised the monster was foolish, and even downright dangerous. He may have agreed in the heat of the moment, but now that he’s able to think clearly, he believes that providing the monster with a companion could mean the destruction of all humankind. If the two of them were to mate and create a “race of devils,” it would be all Victor’s fault.
As I looked on him, his countenance expressed the utmost of malice and treachery.
In this quote from Chapter 20, Victor—as he often does throughout the novel—comments on the monster’s looks. In this example, however, he doesn’t just say that his appearance is hideous. Rather, he goes further by saying that the monster’s unappealing countenance alone is enough to condemn him over. The explanation of this quote in Quotes by Character: The Monster speculates over whether this a fair judgement for Victor to to make or if it’s just Victor trying to shift the blame again for his own mistakes.
You can blast my other passions, but revenge remains—revenge, henceforth dearer than light or food! I may die, but first you, my tyrant and tormentor, shall curse the sun that gazes on your misery. Beware, for I am fearless and therefore powerful.
In Chapter 20, the monster’s response to Victor’s refusal to make him a companion showcases his new motivation: revenge. He once regarded Victor as his creator and father, but now feels nothing for contempt for the man who abandoned him and subjected him to a life of pain and loneliness. With nothing left to lose, the monster tells Victor he is “fearless and therefore powerful,” and swears to force Victor into the same life of pain to which Victor has consigned him.
I shuddered to think who might be the next victim sacrificed to his insatiate revenge. And then I thought again of his words—“I will be with you on your wedding-night.” That, then, was the period fixed for the fulfilment of my destiny. In that hour I should die and at once satisfy and extinguish his malice.
Just after the monster’s departure in Chapter 20, Victor reflects on his threats and comes to the conclusion that the monster means to murder him on this wedding night. The same arrogance that led Victor to attempt to play god is what now causes him to misunderstand the monster’s true intentions. Unable to put himself in his creation’s shoes, and certain as he is that he must be the target, Victor fails to realize what the monster sees as the ultimate punishment: not death, but loneliness. The monster intends to murder the woman Victor loves, forcing Victor to live a life as isolated as his own.