Welcome Laurelin,
I think to truly understand where evil comes from one must ignore the mythology laid out in Genesis. This book was written at a time when man's knowledge was not very advanced. just a cursory study of this book will reveal what a crock of BS it really is.
Brummie may look like a cute cat, but I think he's right about evil coming from the hearts of men. I don't think we need some mystical explanation of a human phenomenon.
We sort of touched on this subject on another thread: