I don't think us non-murderers can ever really understand them. Just one example: Watching those true crime videos I'm thinking, "Damn, guilt would consume me. I would want to be punished".... and I suppose there are a few who do think that way but it's damn few.
I grew up with a guy who killed his stepdaughter. She was 13. Knew him all thru grade school, high school, and some years after that. Not friends, never socialized with him in any way....but NEVER suspected him of being that sick and able to commit such a horrible act. He was tried and put on Death Row here in Calif.... this was back in the late 70's? Just a few years ago he somehow got his conviction reviewed and they found him incompetent. He was released from Death Row and now serves life.
Ever so often my curiosity has gotten to me and I thought I should interview him and write about it. He admits his guilt. Try to figure out how, why, etc. But then I watch these guys on the true crime videos, couple were women, and I don't think they can answer that. Something is missing in them... and more than just empathy.