In the first section of Frankenstein, Mary Shelley offers readers only one side of the rising conflict between the creature and Victor. In Volume I, the story of Victor is "narrated" using a first person point of view. This narrative is provided through Walton, as defined at the closing of Letter IV.
This manuscript will doubtless afford you the greatest pleasure; but to me, who know him, and who hear it from his own lips, with what interest and sympathy shall I read it in some future day!
Therefore, while told by Walton, the story is told using Victor's voice. (This is called a multiple narrative.)
Given that Victor feels it necessary to retell his life, in order to lead up to the Creature's narrative, the first section of the novel is one-sided. Readers are, essentially, only hearing Victor's side of the story.
That being said, readers typically tend to feel sympathy for Victor (regardless of his responsibility for the Creature's creation). Readers hear about the Creature's murder of William and the subsequent execution of Justine (given her charge of guilt regarding William's death).
It is not until the narrative is given to the Creature that readers may come to feel sympathy for the Creature. By hearing his side, readers may come to sympathize with the Creature given his abandonment. The Creature's voice also plays a large role in sympathy gained. Given Shelley fails to describe the Creature in detail, no solid representation is manifested. Some readers may actually forget that the Creature is a monster.
Therefore, in the beginning, a reader may tend to blame the Creature for what he has done. Later, when the narrative voice changes, readers may come to relate with the Creature in a way made impossible when only Victor's voice is heard.