Why does the monster argue that Frankenstein has shirked his responsibility as father/creator and that he should pay for this rejection in Mary...

A dominant theme in Mary Shelley's Frankenstein is responsibility for one's actions, especially responsibility for what one creates. The creature sees that Frankenstein has behaved recklessly by bringing him to life and carelessly by rejecting and abandoning him, actions that have caused him to suffer immensely. Because of his suffering, the creature feels Frankenstein has a moral obligation to take responsibility for the creature's suffering and to ease his suffering, just a little, by creating him a mate.

In chapter 11, the creature begins relaying to Frankenstein his story of suffering. He starts by explaining he had developed his own education by observing the De Laceys in their cottage. While living in the De Laceys' shed, he discovered a leather trunk full of clothes and books, and he began reading the books. One of the books he read was Milton's Paradise Lost. From the book, the creature learned about how God created Adam to be a "perfect creature, happy and prosperous" (Ch. 15). The creature further learned that God protected Adam and educated him through conversation. The creature sees that Frankenstein has behaved in the exact opposite way as God. Instead of creating something that is beautiful and in Frankenstein's own image, just as Frankenstein and all mankind are in God's image, Frankenstein has instead created something so hideous all of mankind is revolted by the creature, terrified of the creature, and desiring to kill the creature. Also contrary to God, Frankenstein himself was so revolted by his own creation that Frankenstein ran from his creation then forgot all about him, leaving him to suffer and fend for himself all on his own. The creature explains that because he was abandoned, misjudged, and hurt so frequently, he was filled with rage and a desire for revenge, a desire that led him to murder Frankenstein's young brother William and frame Justine for the murder. Though the creature has committed murder, he sees himself as ultimately innocent because he sees Frankenstein as being to blame for the rage in his heart, rage that has stemmed from immense suffering. We see the creature's view of his own innocence when, while imploring Frankenstein to have compassion for him and hear his story, the creature says of himself, "Frankenstein, I was benevolent; my soul glowed with love and humanity; but am I not alone, miserably alone?" (Ch. 10). We further see Frankenstein's view of his own innocence when he compares himself to Satan, the fallen angel from Paradise Lost, when he should have been like Adam to Frankenstein:

I ought to be thy Adam, but I am rather the fallen angel, whom thou drivest from joy for no misdeed. (Ch. 10)

When the creature speaks the phrase "no misdeed," we see that he is declaring his innocence. Since the creature sees Frankenstein as to blame for the creature's loss of benevolence towards mankind, the creature feels it is only right that Frankenstein make him a companion so that he might, for once, "feel gratitude toward [Frankenstein] for one benefit!" (Ch. 17).

Frankenstein is at first repulsed by the idea of a second creature, yet he feels compassion for his creature and agrees that, as the creator, it is his responsibility to provide for the creature as much as possible, just as God provided for Adam in Paradise Lost.