In ancient Greek mythology Prometheus was a Titan—a pre-Olympian god. As well as creating humanity out of clay he was also renowned for defying the gods by stealing fire and giving it to humans. For this serious crime he was punished by mighty Zeus, who sentenced him to be chained to a rock, where his liver would be pecked out by an eagle every day before growing back overnight, only to be pecked out again the following day. For his defiance of the gods Prometheus was traditionally regarded as something of a hero, a champion of the human race.
In writing Frankenstein, Mary Shelley was strongly influenced by the Prometheus myth. Indeed, the subtitle of the book is "The Modern Prometheus". Far from seeing Prometheus as a hero, however, Shelley regarded him as a villain responsible for corrupting humankind, and that's clearly how Frankenstein is presented throughout the story. In creating his Monster he has appropriated the kind of power traditionally associated with God. By playing God, Frankenstein unleashes a great terror upon an unsuspecting world.
Just as Prometheus, in the eyes of Shelley, corrupted humankind, so too has Frankenstein in creating his Monster. Frankenstein wants to create a race of creatures, just as his mythological predecessor created the human race out of clay. The consequences of both these acts of transgression—defying the gods, or, in Frankenstein's case, the Judeo-Christian God—have been damaging, to say the least.