How we cite our quotes:
"Frankenstein! you belong then to my enemy--to him towards whom I have sworn eternal revenge; you shall be my first victim." (16.30)
Notice that the monster thinks of William as "belonging" to Frankenstein. He doesn't seem to see much difference between killing William and burning down De Lacey's cottage—all's fair in revenge and war.
Elizabeth read my anguish in my countenance, and kindly taking my hand, said, "My dearest friend, you must calm yourself. These events have affected me, God knows how deeply; but I am not so wretched as you are. There is an expression of despair, and sometimes of revenge, in your countenance that makes me tremble. Dear Victor, banish these dark passions. Remember the friends around you, who centre all their hopes in you. Have we lost the power of rendering you happy? Ah! While we love, while we are true to each other, here in this land of peace and beauty, your native country, we may reap every tranquil blessing—what can disturb our peace?" (9.8)
For a novel about creating life, Frankenstein doesn't talk much about religion. Here, though, we get a glimpse of an alternative moral structure: Christianity, which is all about turning the other cheek and avoiding revenge. Would the novel have turned out differently if Victor had listened?
When I thought of my friends, of the mild voice of De Lacey, the gentle eyes of Agatha, and the exquisite beauty of the Arabian, these thoughts vanished and a gush of tears somewhat soothed me. But again when I reflected that they had spurned and deserted me, anger returned, a rage of anger, and unable to injure anything human, I turned my fury towards inanimate objects. As night advanced I placed a variety of combustibles around the cottage, and after having destroyed every vestige of cultivation in the garden, I waited with forced impatience until the moon had sunk to commence my operations. (16.12)
Well, burning your friends' cottage is definitely one way to take revenge. (Also, can we just point out that it is super racist that the monster keeps referring to everyone except Safie by name?)