I think he killed her because she was a sort of perfectionist and pissed him off past his level.
I still wonder about those two murdered women found around the same time.
Did that have anything to do with this?
I disagree. They were in a tiny van that needed to be kept tidy. It was stated early on that he was controlling and Gabby often stayed with her friend when Laundrie fought with her. Friends from their HS days warned her to stay away from him. The body cam footage and 911 call revealed that he had her keys, phone and backpack. We also saw his temper at the Merry Piglets restaurant. The van and credit/debit cards were hers. I think he found a naive girl that couldn't see him for what he really is and he was along for the ride.....