What do the numerous murders and much rampant violence throughout the novel portray?
Why would Shelley incorporate so much brutal violence into the novel?- what does it come to teach us about the characters, etc.?
I think the violence in the novel is done for two primary effects. The first is that Shelley is striving to offer genuine criticisms of the Neoclassist theories that preceded her and the Romantic theories, with which her husband (Percy) was enamored. Part of the Neoclassist approach of which she was against was a revering of science and scientific inquiry. Victor is a man of science, wedded to it, and committed to the idea that science can reveal truth. The fact that Victor created the monster was a testament to what he felt was the glory and power of science and rational thought. When that creation is responsible for death and violence along with Frankenstein failing to take responsibility for it, Mary Shelley is critiquing the belief that science is perfect, flawless and something whose sole quality is the betterment of society and quest for truth. However, Mary Shelley also critiques the Romantic vision of the truth, which succeeded the Neoclassist thought. In this setting, truth is not achieved through science, but through individuality, seclusion, isolation from society and the belief that with enthusiasm and freedom, truth can be recognized. Shelley critiques this notion as well when Frankenstein retreats to the countryside and isolates himself from his responsibility. When Frankenstein is horrified at what he has created, he leaves it, using his purest sense of freedom. Shelley is suggesting that with his embrace of freedom without consequences and not accepting his responsibility to society for his creation, again, death and destruction are not far behind. In making the monster a violent one, Shelley has criticized both schools of thought by showing how terrible extreme following of one can be.
Another school of thought is that Shelley wants to show the need for social responsibility. Frankenstein does not show any sense of ethical or moral responsibility for what he has done. In displaying the violence that results from the monster, Shelley shows us that human beings must adopt a framework of responsibility for their actions and their creations. When one abandons this sense of duty, bad things follow.
Much of Shelley's use of violence is simply in keeping with the Gothic elements of Romanticism. Gothic works possess mysterious settings, dark deeds, and tortured souls. While many high school students think of Poe as insane (he was a little strange), much of his writing stems from what was popular for his time. The British Romanticists such as Shelley came before Poe and established the violence and mystery associated with Gothic writing.
So, while Shelley does include the senseless murders to fit her Gothic genre and to entertain, she also had additional motivation for doing so. The author grew up in a setting where learning, even on the part of women, was encouraged. She often sat in on conversations between her father and scientists/philosophers of the day. Readers can see their influence on Shelley's writing. She uses this insatiable quest for knowledge, both on Victor's and Walton's parts, to illustrate the dangerous consequences of obsession.
Victor, of course, believes that he is guilty for much of the violence because he created the murderer. The Monster's murders are perhaps an exaggeration of how science can go wrong, but they do make a point. Shelley, as a Romanticist, would have been more interested in showing how humans should go to nature for answers about life. She was writing against the modern thinking of her day that proposed that pure science could answer man's questions about the meaning of life, etc.