1. As the novel progresses, Victor becomes more like his creation. At first, he is horrified at the Monster's appearance and his own "handiwork." However, as the killings multiply, Victor comes to view himself in a horrific light. He realizes that he, like his creature, is a monster because of what he set loose upon the world. At the beginning of Chapter 9, Victor craves isolation just as the Monster does, away from the probing eyes of mankind. He states,
I shunned the face of man. . . . solitude was my only consolation--deep, dark, deathlike solitude.
When the Monster kills Elizabeth, Victor becomes even more like his creation--he is fueled by revenge. As he visits his father's, brother's, and wife's graves, he promises,
For this purpose will I preserve my life: to execute the dear revenge will I again behold the sun and tread the green herbage of earth (Chapter 24, page 190).
The Monster wants revenge upon Victor for abandoning him, and Victor wants revenge for the loss of everything dear to him. In the end, Victor and the Monster change roles; Victor, once the pursued, pursues his creation.
2. Many would argue that the Monster is truly the book's protagonist. Almost all of the novel's conflict is centered upon him, and the two pivotal moments in the book--the destruction of the female creature and Elizabeth's death--are generated by the Monster. However, Victor does represent a true tragic hero and a Romantic hero. He falls from a high position (noble birth, well respected, etc.); he possesses a tragic flaw--hubris and an insatiable thirst for knowledge; and he demonstrates a tragic realization while on his deathbed on Walton's ship.
3. I personally believe that Shelley intends for Victor to be the novel's protagonist, but also for the reader to see the creature not as her work's antagonist but as a mirror character for Victor.