The first of these quotations is spoken by Victor. He has just returned to Geneva, having learned that his brother, William, has been murdered. In the light momentarily provided by lightning, Victor sees a gigantic creature in the distance and knows, instantly, that it is the creature he created two years prior.
Even before he encounters the creature directly, he is ready to assume the worst of it. He calls it a "filthy demon," and he believes the creature killed William. It is significant that Victor first sees the creature when lightning strikes because lightning is associated with Zeus, while Victor is called "the modern Prometheus" in the book's subtitle. In Greek mythology, Prometheus is able to thwart Zeus, refusing and angering the Olympian, but Zeus is also plenty strong enough to make Prometheus suffer for a long time. The fact that these two immortals are pitted against one another does not bode well for the relationship between the creature and his maker, Victor, who are respectively associated with them.
The second of these quotations is located on page 116 and is spoken by the creature. He intends to tell Victor the story of his life since Victor created (and abandoned) him. It is notable that the creature associates the telling of his own story with the sun being "high in the heavens," because Zeus is the god of the sky, so powerful with his strength and will. The creature will go on to assert his power over Victor, insisting that he will make Victor suffer if Victor does not give him what he wants. This is precisely what Zeus did when he chained Prometheus to a mountain as punishment for refusing to give him the information he sought.