What is Mary Shelley's attitude toward the evil nature of the Creature? Is he entirely to blame for it?

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.