On Chapter 24, (part three) Victor explains his behavior with the monster.
Basically he explains that he has taken the time to go back tp analyze his actions, and that he feels partially responsible for them, although he was acting "under a fit of enthusiastic madness" that led him to try for an ambition that was too big even for him. He even claims that he would have done anything to assure the creature's "happiness and fulfillment" , but that since the creature ended up being so evil and killling all that Victori loved, now he wante to avenge it.
Furthermore, Victor even plays himself as a good guy by saying that, out of this same feelings, he refused to create a partner for the creature. Hence, he was almost saying that it was his god kidness what saved the world from a culture of monsters, and it was all out of scientific ambition and good plans for the creature that he embarked on his experiments. We know that such thngs were all lies.
The quote is below:
During these last days I have been occupied in examining my past conduct; nor do I find it blamable. In a fit of enthusiastic madness I created a rational creature, and was bound towards him, to assure, as far as was in my power, his happiness and well-being. This was my duty; but there was another still paramount to that. My duties towards the beings of my own species had greater claims to my attention, because they included a greater proportion of happiness or misery. Urged by this view, I refused, and I did right in refusing, to create a companion for the first creature. He showed unparalleled malignity and selfishness, in evil: he destroyed my friends; he devoted to destruction beings who possessed exquisite sensations, happiness, and wisdom; nor do I know where this thirst for vengeance may end. Miserable himself, that he may render no other wretched he ought to die. The task of his destruction was mine, but I have failed. When actuated by selfish and vicious motives I asked you to undertake my unfinished work; and I renew this request now when I am only induced by reason and virtue.