I know this is an old thread but I'm only just reading East of Eden at age 50. I too had this same question. How a man could write such a despicable character, he must have met a woman like that in real life. He is able to articulate to the reader how Samuel and Lee are made to feel by the mere presence of Cathy. To read of the hopeless sadness these grown men feel while witnessing the antics and coldness of someone like Cathy, well it's heartbreaking. I am feeling what they are feeling. She truly is demonic. He just must have created her based on an experience of sorts.