In Mary Shelley's Frankenstein where does evil come from?

As Mary Shelley's novel, and Boris Karloff's classic film portrayal clearly show, Frankenstein's monster is pathetic. He was born with the capacity for good and bad, but circumstances brought out the bad in him. Horror films since Karloff's 1930s Frankenstein have tended to show the monster as a bloodthristy animal, but the real story is actually more interesting. If you realize that the creature had the potenial for good as well as evil.