I think that this question goes to the very heart of the novel. It is also a source of a great deal of debate with those who read it. Strong cases can be made for Shelley siding with Victor as opposed to siding with the monster. Its ambiguity is what contributes to the greatness of the work. For my bet, I would say that Shelley sides with Victor having to shoulder most of the blame for the results of his creation. The idea of Victor running away, abandoning his responsibility, and not being able to fully embrace the ethical dimensions of his creation are of vital importance to assessing how Shelley views the level of responsibility in the predicament of the monster.
One of the ironies of Shelley's Frankenstein is that in spite of the atrocities the monster commits, readers tend to view him with more sympathy than they do Victor Frankenstein.
Diane Johnson, in her introduction to the Bantam Books edition of the novel, writes:
The moral ugliness of Victor's lies is expressed in the monster by his hideous countenance; what is inside Victor is exteriorized, and of course excites universal antipathy among the other characters in the novel, though it strangely invites the affection of readers, who are usually agreed that the monster is sympathetic however horrendous his deeds. (xvi)
In other words, Victor's immoral decisions concerning the creation of the monster are present in the monster's appearance, but readers tend to blame Victor, not the monster. Blame is further given to Victor in the novel by the nature-nurture theme. The monster becomes what he is because he is not nurtured by Victor. Left on his own, natural man can become a monster. Victor is responsible for nurturing his creation, and he fails that responsibility.
Finally, the monster is very much the alter-ego of Victor. The monster is Victor's wild, untrained side.
There is much evil to go around, and if anything, Victor is more to blame for events than the monster.
In my opinion, the monster's nature is caused by two things. First it is caused by the hubris of human beings. Second, it is caused by their bigotry. So I think that she is blaming humanity for the monster's evil deeds.
Hubris is to blame because hubris makes Frankenstein create the monster. He feels that it is acceptable for him to make the monster just because he can and because he wants to discover the secrets of nature.
Bigotry is to blame because the monster seems to be just fine until he is rejected by everyone he meets. The main reason for this is his appearance. By rejecting him, people cause him to turn bad.
I believe Shelley purposely creates ambiguity concerning the source of evil in the creature so that the reader is left to ponder the true source of the evil. Perhaps man in his ultimate desire to attain greatness beyond that of mankind is to blame? Perhaps the nature of the creature is symbolic of the dark side of man left unchecked? After all, the creature is the product of man. Her goal not to make perfectly clear who is to blame - Frankenstein or the creature - allows ongoing debate for years after the novel's publication, which may be one of the many reasons Frankenstein has been considered one of the all-time greats!
In the book Frankenstein, Mary Shelley presents the creature as a doomed man left to suffer from social isolation who craves love and companionship. She does focus as much blame on the creature's evil nature. She actually presents his evil nature more in terms of social rejection and emotional pain. It would stand to reason that if the creature had not been able to develop socially with guidance that he would not have the ability to manage his anger and would react violently.
Victor gave the man extreme physical strength and power when he chose to make him very large and picked the body parts. He also condemned the creature by his lack of concern for his looks or emotional needs. He then rejected his creation and left him to fend for himself. The creature is sad and lonely. He becomes desperate for affection. Shelley was very empathetic towards her creature.