I will revenge my injuries; if I cannot inspire love, I will cause fear, and chiefly towards you my archenemy, because my creator, do I swear inextinguishable hatred. Have a care; I will work at your destruction, nor finish until I desolate your heart, so that you shall curse the hour of your birth.
This quotation by the Monster comes from chapter 17. After failing to persuade Frankenstein to make him a mate he vows a terrifying revenge on his creator. The Monster gamely tried to reason with Victor, but all to no avail. The Monster is truly miserable, shunned, and despised by society. He's become convinced that the only way he can ever be happy is if he has a mate to stand alongside him through all of life's many adversities.
But Victor won't play ball, and so the Monster threatens to destroy him. This threat tells us a lot about the Monster's character. Although in many respects he's quite a sympathetic character, he also retains the monstrousness that has already led him to commit murder.
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."
This quotation, from chapter 16, shows the Monster exulting in having just committed a brutal murder. As we can see, he feels a real sense of power as a result of his wicked deed. For the Monster, going around causing havoc and murdering people gives him a sense of control over his life, something he's never really had before.
Frankenstein may be his creator, but the Monster's now taken on a life of his own. He's broken his programming, so to speak, and now feels liberated, that he can do anything he likes. Just as Frankenstein derived pleasure from creating, so the Monster derives pleasure from destroying.