Frankenstein takes the form of a nested series of second-hand narratives which, in the end, all originate in men. Walton, Frankenstein, and the monster, the principal narrators of the story, are all male. The story of the ruin of the De Lacy family, as related by the monster, occurs entirely through actions and decisions made by Felix. Women in the novel simply exist to be blown about by emotional whirlwinds and blindly go along with what their male relatives tell them: they cry, they faint, they are imprisoned and killed, and all along they simply dote after their loved ones without ever putting up an argument or a fight. But why would Shelley, an author raised in one of the most radical feminist and leftist households of her time, tell the story this way? I believe it was a deliberate stratedgy designed to illustrate the ways in which women are entirely left out of patriarchal society, even as they suffer because of it.
As a representative of both Enlightenment rational science and the Romantic ideal of the towering individual capable of godlike artistic creation, Frankenstein allows Shelley to critique the ways in which both movements leave women out of the supposedly radical new worlds they attempt to create. Frankenstein, in his massive ego and self-obsession, can only think of himself even as Justine is condemned to death: ". . . none ever conceived of the misery I then endured." He says this again and again in the novel - that no one, absolutely no one else on earth could possible understand his suffering. He has direct knowledge of four people who have been murdered during these events, and yet follows convoluted trains of thought to rationalize why his suffering is even greater than theirs.
This egotism is especially shocking when one considers the extreme negligence that Frankenstein displays in allowing the murders of Justine and Elizabeth to occur. The murders of Henry and William occur without his foreknowledge, apparantly on impulse of the monster, but the murders of the women took place after Frankenstein had ample time to prepare a defense. Justine is sentenced to death for reasons Frankenstein knows to be false, and yet he merely gives a token speech about her good character rather than ever actually putting an effort in to save her. He justifies this by saying that he can't reveal the story of the monster, no one would believe it, and so on and so on, but when he himself is accused of Henry's murder in Ireland, he immediately tells the entire story to the magistrate in an attempt to save himself. How quickly he abandoned his rationalizations when it came time to save his own skin!
Similarly, Frankenstein is sure the monster's threat to "be with him on his wedding-night" was a threat on his own person, and doesn't for a second consider defending Elizabeth or keeping her safe. It's almost comical the way he arms himself to the teeth in preparation for his epic final confrontation with the monster, and then at the first opportunity leaves Elizabeth entirely alone in the bedroom to be instantly sacrificed! This could easily be one of those moments when the audience shouts at the screen in a cheesy horror film, but I think Shelley did this purposefully to demonstrate the ridiculous degree of Frankenstein's self-absorption, and therefore illuminate, through exaggerated effect, the ways in which her society downplays the importance of women as human beings.
Meanwhile, the account Frankenstein (via Walton) gives of these women is as absurdly passive window-dressing. Felix wins his wife as a prize for saving her evil, conniving father from yet another wrongful death sentence, and at no point does she demonstrate either a devotion to her father or a desire for any life other than being dragged around by men. Justine is hyperbolically thankful that Frankenstein believes her innocent, but in the end confesses to the crime anyway and goes like a lamb to the slaughter, never fighting for her life in a significant way. And finally, Elizabeth's major contribution to the novel is in her letters to Frankenstein, which essentially say that she wants nothing but for him to be happy and that he should feel free to abandon her if he feels like it. Despite the fact that Frankenstein supposedly has a heart "overflowing with kindness and virtue" and has lived his whole life devoted to these women, he doesn't actually take them into account as people at any time in the narrative, and his main concern is his own guilt and depression over being responsible for their deaths, to the point that he completely fails to make any attempts to save them.
Frankenstein's attitude toward women is revealed in its most shocking psychological qualities exactly at the moment of the creation of the monster: upon seeing the horror of the thing he has created, he immediately runs away to lock himself in his bedroom and sleep it off. While sleeping he dreams of seeing Elizabeth in the street, but, after trying to embrace her, she turns into the worm-ridden corpse of his death mother, and upon waking up he finds the monster crawling about with a childlike grin on its face and making sounds like a newborn baby. All this action takes place during a single paragraph, which implies that Shelley intended for it all to be connected. Frankenstein is horribly conflicted about his promise to his mother, on her deathbed, to marry Elizabeth, who has been raised with him as his sister for his entire life. The two images - of physical love and his mother's corpse - are intimately connected to him, and essentially he has invented a way of procreating, of creating children who are loyal to him, entirely without the need for women.
Shelley's striking use of horrific imagery here serves both to convey psychological motivation of Frankenstein as a character, but on a larger scale also forms a critique of male-dominated scientific endeavor: she clearly fears that, in a society that discounts women entirely and refuses to even acknowledge them as human beings, men will eventually attempt to use science to make women completely obsolete. The repressed, patriarchal society that Shelley lived in essentially valued women as nothing but objects which had no value except to provide visual pleasure for men and, eventually, provide the biologically necessary service of reproduction. One of the major warnings in Frankenstein is that an advanced technological society which doesn't view women as human beings with value may eventually attempt to write them out of existence entirely.