Frankenstein's Creature feels empowered after killing William and framing Justine. He has nothing against either of them, as people. In fact, he originally planned on having William for a friend, before he learned that the kid was related to Victor. However, the Creature realizes that hurting these two will hurt Victor, the one individual most responsible for the Creature's outcast place in the world.

The Creature projects its rage against mankind onto Justine. He tells Frankenstein that he placed the locket on her to frame her for the crime. He feels he is receiving retribution for how he will never have a woman look upon him with compassion. By allowing Justine to be framed and executed for a wrong she did not commit, the Creature is getting vengeance for being treated as a monster even before he committed a single wrongdoing.

Immediately after killing William, the creation is filled with a sense of joy and power.  He realizes for the first time that he, too, has power.  He has the power to inflict pain and misery on man, specifically Victor.  He says, "...my enemy is not invulnerable; this death will carry despair to him, and a thousand other miseries shall torment and destroy him."

The creation tells Victor how he used the miniature portrait to frame Justine (by placing the portrait in one of the folds of her dress) because of his rage from realizing that no woman would ever look on him softly and with tender feelings because of his appearance. He chooses to punish Justine symbolically for all women. "...but she, shall suffer; the murder I have commited because I am forever robbed of all that she could give me, she shall atone."