The monster explains how this happened in chapter 16 as he reflects upon how those events transpired. Initially, he did not seek to harm the young William, whom he describes as a "beautiful child" who disturbed the creature's rest as he sought respite from mankind in those early days. As he watched William, he believed that he was too young to have been prejudiced by the world and that he could thus train William's young mind to accept the deformities of the monster, thus building a natural friendship with the boy. Instead, when confronted, William screamed in horror and called him a "monster" and threatened to tell his father, Mr. Frankenstein.

This name incited an inner rage within the creature, and he acted swiftly, grabbing young William around the neck and killing him quickly. Instead of looking upon this murder with regret, which might suggest a being capable of developing a moral compass, his heart swells in "hellish triumph," thrilled that he, too, can create desolation.

The murder of William, then, is partially directed at inflicting pain on Victor Frankenstein but also originates from a quick and deep rejection from an innocent person whom the creature feels is his best bet for acceptance. He feels that if a young child, too young to have been taught the world's prejudices, cannot accept him, he is doomed to a life of alienation, and he will thus inflict as much pain upon this world as possible.

He frames Justine for the murder because he sees her as "one of those whose joy-imparting smiles are bestowed on all but me." Justine embodies a joy reserved for everyone human, and the monster sees in her all the rejection that he will face. He frames her simply because he seeks destruction, and he realizes that he has the power to accomplish it. He wants to see her suffer because he suffers. Justine is framed because she symbolizes the joy of mankind which the monster realizes he can never know.

The Monster's brutal murder of William and his subsequent framing of Justine is a sign that he's becoming more human yet no less monstrous. As the Monster starts to learn more about the world, he realizes just what merry havoc he can cause, especially to those who cross him. As the Monster isn't human, he is beyond the bounds of society, with all its norms, laws, and values. Under the circumstances, then, it makes perfect sense for him to commit an act of murder and have someone else take the fall for it.

The Monster is motivated primarily by revenge. He's grown to hate Frankenstein for disowning him and for not giving him the companion he so desperately craves. But as Frankenstein is the only man alive who can do this for him, the Monster spares his life, choosing instead to get at his creator through his nearest and dearest. In killing William and framing Justine for the crime, the Monster hopes to intimidate Frankenstein into doing what he wants.

The creature kills William and Justine to get revenge against Victor for creating him and then wholly rejecting him. He has been hurt by lack of love and acceptance, and he wants to strike back in return, out of his pain and anguish. As the creature says to Victor:

Why did you form a monster so hideous that even you turned from me in disgust? God, in pity, made man beautiful and alluring, after his own image; but my form is a filthy type of yours, more horrid even from the very resemblance. Satan had his companions, fellow-devils, to admire and encourage him; but I am solitary and abhorred.

His murder of William is directly in retaliation for Victor's hatred of him. With Justine, it is slightly more complicated. The creature reacts against Justine not only because Victor, his creator and "parent" rejected him, but because others to whom the creature has reached out, even seemingly good and kind people, have also reacted in horror to him. Therefore, when the creature comes across Justine, he wants to kill her not only because of her relationship with Victor, but because he knows she would abhor him were she to awaken.

In Mary Shelley's novel, Frankenstein, the creature kills Frankenstein's youngest brother William, and frames another member of their household: Justine.

When the creature sits with Frankenstein and tells his story, he relates his trip to Geneva, coming by chance across young William. The creature believes that a child not having learned to be fearful of the world, might be a good companion for him. He tells William he will take the boy who will not see his family again, but the boy shares his identity as a Frankenstein, threatening the creature with his father's protection. Once the creature realizes this boy is related to Victor he says:

Frankenstein! you belong to my enemy—to him towards whom I have sworn eternal revenge; you shall be my first victim.

In order to punish Victor, the creature kills his young brother. In doing so, he discovers and takes the miniature portrait of his mother (Caroline) that the boy carries, away with him. Soon he comes upon Justine, asleep in the barn. She is not as beautiful as the woman in the portrait, but even as he thinks this, he knows that should she awake, she would reject him in horror as others had done.

Thus she would assuredly [denounce me if she] beheld me. The thought was madness; it stirred the fiend within me—not I, but she, shall suffer; the murder I have committed because I am forever robbed of all that she could give me, she shall atone. The crime had its source in her: be hers the punishment!

Knowing something of the law from listening to Felix, the monster plants the portrait in the folds of Justine's dress so she will be accused of the murder of William.

In essence, the creature kills William because he is related to Victor, and he frames Justine with the sense of power he realizes he has to bring havoc to the world around him: especially to her because she represents all he cannot have.