My thoughts about that horrible night. Just one of my many theories. Based on nothing but snippets of what life in the trailer park might have been like and could have led up to.
I think that mom and dad sent the girls to stay for few days - why, I'll leave that up to speculation, hopefully disclosure in the future.
I'm thinking MP was still at the "grooming stage" (Ali first, since she's more to his pre-teen preference). That's why the genuine excitement from the girls to stay with Uncle Mike. He's fun, lets them play games, maybe stay up later watching TV - things they don't get to do at home.
He knows mom has issues, is overwhelmed, having an especially bad week and will be less likely to interfere this particular weekend. She needs help, so he volunteers to help out and keep the girls. It's divine intervention - these few days are the time to make "his move"... He's been waiting for just the right time and thinks she's ready.
But he misjudged her. She's not the submissive little girl he'd been hoping for and makes a break for it, unfortunately, she's not fast enough. He subdues her - kills her.
Maybe it's just wishful thinking, but I'd like to believe that she was NOT sexually abused/assaulted by him before she died. I think she still would have died, because it sounds like she had enough spunk in her to tell someone, but I'm hoping she wasn't abused.
I also don't think he dismembered her to hide her body. He folded too easily. I think he was angry at the rebuff from her, and in a fit of rage killed her and cut her up. Maybe he'd had enough - frustrated the grooming tips from JL didn't even work for him! I think he then decided he wanted to be caught for whatever reason.
Why keep pieces of her with sisters there and the police sniffing all around? If he left those little girls in the middle of the night as he said to go get a cigar - he could have dropped off plastic bags all over and throughout town.
Maybe he thought he'd never been important to anyone before - maybe felt like he was a "mistake", ignored by mother-who wasn't ready to be one when she had him, not acknowledged by bio father, women, who knows. Maybe there are more bodies out there?
I think he's getting the attention he's always wanted now though.
JMHOO