I understand what you're saying and if AC were "just" a paedophile I would agree. However, I believe this horror of an individual enjoys inflicting pain - physical and emotional - and what better way to hurt Genie further than to harm, or kill, her remaining child - regardless of her age? I imagine he would also get great enjoyment out of the extra fear and terror the girl/woman would feel, knowing that her attacker had previously brutally raped and murdered her sister.
JMO but I think it's dangerous to try and put this subhuman into a specific category - based on some of the comments in the appeal report I'd say it's the terror, and the power to inflict that on others, that is the real motivation, not paedophilia.
I think the crime he committed was simply a combination of the most extreme things he could think of combining into one offence - IMO this is confirmed by his reaction to learning that the post mortem showed he "failed" in his desire to include necrophilia. That must have really taken the shine off the crime for him and I personally think he will be planning what he'd like to do to rectify that whilst he's incarcerated.........so no, I really don't think Genie is overreacting. I think this very dangerous individual is way past rehabilitation, but he might well be able to fake it enough to get out for one more horrifying offence.
JMO, IMO etc