Inside us all burn passions, or daemons, and if not handled properly, the battle is lost. Passions expand into obsessions and obsessions expand into psychoses, resulting in unacceptable societal behaviors. This is the route Mary Shellys Victor Frankenstein takes, and he loses his battle by becoming enslaved to his ever-demanding desires.
The monster, as always, is the child of the mind, its darkest thoughts. Usually, the monster is controlled. We are taught constraint as children at school or home not to act on our darkest desires, but Victor is spoiled.