I don't want to present myself as any sort of sleuth, I'm not. Seriously, I don't read fiction, and I'm not one of those people who gets all excited about so-called "mysteries" or "true crime detective" stuff.
However, you work in a laboratory for 30 years and you pick things up. You can pick up when domestic violence is occurring and (boy is this controversial) when a domestic violence report is "fake" as well. And we all deal in probabilities, but there's also a level of common sense.
For example - set aside even some of the most obvious evidence in the murder of Nicole Brown Simpson (you know.....her blood in his Bronco, his blood at the scene, OBVIOUS stuff). The knife murder with its sheer brutality suggests an extreme form of personal hatred. If Dershowitz's red herring about Colombian drug dealers had had any level of veracity, either she would have been popped quickly and left out in the open sun to be found as an example OR - assuming the neck gutting theory - that wouldn't have been done there at her house, although she might have been found there. Her murder was someone full of rage that made it personal. Same with Brian Laundrie killing Gabby Petito.
Or the infamous Buford Pusser ambush. Now ask yourself an obvious question: does a rolling ambush with three hit men in the dark hours of the morning on a road he was least likely to travel to investigate the alleged trouble - and then they came back and fired more rounds BUT STILL DIDN'T KILL HIM......does this even sound remotely believable? But Hollywood can sell anything.
I think you're right and this has sounded to me sorta similar to the case (but with a different reason) of Pierre Laporte, a Canadian politician kidnapped by a group of leftist terrorists (FLQ) to be held to force the release of some prisoners. They had NO INTENT to kill him (see October Crisis 1970). But he tried to escape, injured himself, and they chose to finish him off rather than take him to the doctor. They had kidnapped another guy a few days earlier, and he wound up getting released.
Dead bodies are not any good for ransom, and you have to assume they wanted something.
But their failure to prove her being alive at the early stage told me the mother was probably already dead.