I think Casey and Scott Peterson are more similar...they both desperately wanted out of a confining situation and in their sociopathic minds without a conscience, murder was as viable an option at any other and they could easily live with it. It was a simple step by step decision without emotion to free themselves to a future they preferred. The killing was merely a means to an end.
This chick is way more evil, diabolical. She enjoyed the murder, enjoys the stand, the attention, the deceiving the infliction of pain, the hurting other people like Travis' family including his sweet innocent grandmother. She enjoyed the killing of Travis making it as INhumane as possible. It was far from an extermination...it was a sadistic violent bloodbath that she enjoyed every second of (and probably relives it daily with pleasure). To me, she's as demonic as anything I've ever seen.