Yes, ITA with everything you said.
I was talking with a friend about this case last night, and how horrible it was. He said, you know, dismemberment is a really horrible thing, especially when we are talking about a child. Horrible doesn't even feel like a strong enough word to describe what it makes you feel. But, think about what's on TV... and think about the person who did this. Dexter dismembers a body every time he kills... which is, apparently, every episode? (I've only watched a few episodes on Netflix and didn't like it, so am not sure what the show is about...). He said, as sick as it sounds, that dismemberment is probably not such a big deal to people who obviously have so little regard for human life in the first place. I think he's onto something... anyone with an ounce of empathy would never abduct a child to begin with. So we can't approach this from a place of empathy because it just doesn't fit... the guy who did this does not think like us. So I think my friend was onto something, and I think that, for the person who did this, the choice to dismember was a pragmatic choice more than a sadistic choice... if that makes sense.
ETA... just to clarify that I don't mean to claim that the person who did this did so because he watches Dexter, but just to comment on the general knowledge of dismemberment in popular culture, as well as knowledge of forensics investigations in popular culture and how these sorts of things, but particularly dismemberment, tend to, through familiarization via popular culture, can maybe seem like pragmatic choices to those who commit murder.