There was none among the myriads of men who existed who would pity or assist me; and should I feel kindness towards my enemies? No: from that moment I declared everlasting war against the species, and, more than all, against him who had formed me, and sent me forth to this insupportable misery.

The monster continues his defense of his violent murders in Chapter 16 stating that he was merely defending himself after humanity declared war on him. Read more about this quote in Quotes by Character: The Monster.

I gazed on my victim, and my heart swelled with exultation and hellish triumph; clapping my hands, I exclaimed, 'I too can create desolation; my enemy is not invulnerable; this death will carry despair to him, and a thousand other miseries shall torment and destroy him.

In Chapter 16, the monster tells Victor that he murdered William after a decisive and horrifying shift in his perspective regarding humanity. His longing to connect with and befriend humans was transformed into an obsession with destruction and vengeance. He is thrilled that he killed William because he knows it will hurt Victor terribly. At this point, he no longer seems to have any concerns about morality or goodness. The isolation, rejection, and abuse that he’s endured have warped his desire for connection into a thirst for power over humankind and the desire to inflict suffering on his creator.