I would agree with you except for one thing. To the best of anyone's knowledge, JonBenet's killing was a one-time thing. In other words, there isn't a sexual serial killer out there who has committed other murders that had the same or similar MO to the Ramsey killing.
If it's done only once, rage seems the more likely motivating factor. Again, I readily admit I could be wrong. What I've described is how the autopsy photos and report painted the picture of the killing in my mind.
I would agree that her case is unique. I think the killer is particularly proud of this case. I'm not saying it's Karr. I'm not convinced of that yet. There are so many unsolved cases that I'm not so sure he's not done it again. Child molester's don't alway use the same M.O. Westley Alan Dodd is a good example. Amber Haggerman's killer has also never been caught and no other cases quite match her's either.