East of Eden: Cathy

We had a long class discussion about Cathy a few weeks ago, and we were trying to figure out why she is so utterly evil.  Steinbeck called her a monster, and believed she was missing something, but didn't really know what.  I think she's missing timshel.  She is totally incapable of seeing the good things in life, so how is she supposed to realize she has an option other than her depraved lifestyle? What I don't understand is how she lost that element- her family wasn't abusive and her childhood was normal besides her eccentric habits.  How does someone lose their ability to see the good in people and in life?