What is the author's overall argument in Frankenstein?
Frankenstein has many main themes and arguments, and one of the main arguments is that people should not attempt to subvert the natural order of life and death, whether it be from God's plan or from nature itself. Either way, Mankind is not meant to be creators of life except through natural childbirth, and the act is so arrogant that it causes death and destruction despite any well-meaning intentions.
"Seek happiness in tranquillity and avoid ambition, even if it be only the apparently innocent one of distinguishing yourself in science and discoveries."
(Shelley, Frankenstein, gutenberg.org)
Because Victor Frankenstein was so obsessed with replicating what he saw as the science behind creating life, he became an outcast. He denied himself personal interactions and relationships, and finds that he is terrified by the reality of his creation. His cowardice proves that Mankind is not meant to subvert the natural order of life and death, and his condemnation first by society and then by his Monster itself shows that he never considered the consequences of his actions.