What makes a killer?

This morning I heard about a woman who dropped her baby from a parking garage and the first thing I asked myself was, "How could she do such a thing?" Were you as glued as I was to the Casey Anthony trial? Again, "How could she (if she did, right? after all, she was acquitted of murder)-- How could she do it?"
I posed that question as the theme of my book, The Dark Before Dawn and during my research, I was astounded to hear about a new theory weaving its way through the psychology world. There's a book out called "Hope for Humanity" and a fascinating website:http://www.humansolutionsnow.com/(look up "generational trauma") which address this.
I was always a believer that environmental issues played the biggest part in turning someone into a sociopath or even a killer -- bad parenting, a childhood trauma, etc. The study of epigenetics turns that on its head because it means that the trauma caused by anything, a war, a tsunami, can imbed catastrophic impressions into our DNA and thus get passed along in our genes. So are killer predisposed to killing because it's in their genes? I would love to hear what you think.