I've said this before, and I still think it is entirely possible that:
1) There are situations where all three could have been targeted, either because all three knew something (or someone thought all three knew something).
2) There were a lot of young kids involved. And you know how rumors get spread among high school age kids. We already know a lot of rumors mentioned here that sound like things high school kids would say - things like so and so is pregnant, so and so is mad at whoever, etc.
3) I think it's possible there was a plan in process, but something happened that particular night which forced action. When all things were considered, this was probably a riskier night than many others for this action. Something someone (a guest at one of the parties) might have overheard.
4) I think the actions of some people close to the missing three, whether they knew it or not, is what set the events of the evening in process.
5) Given that type of involvement, I can see why the ancillary players who got the wheels rolling may have remained silent as they might be considered accessories.
6) The women had to be removed. Were they killed at the house, it would have been easier to trace back through the chain to figure who did it. In the end, I think the motive was silence, not revenge.
What I can't see is how there is no real sign of struggle at the house. A broken light globe makes no sense unless the only struggle was outside the house. Otherwise, there would be more disarray inside the house and stuff probably broken.
As for other theories, my opinions:
Bartt was a convenient suspect, given some of his past. But I'm confident he wasn't involved. Even if he'd gotten someone to "do him a favor", someone would have probably rolled over by now - selling him out for reduced time. I think anyone involved that's already in jail isn't rolling because they don't have a "big fish" to roll on.
I don't buy a sexual predator. Many times those crimes are impulsive. The actual events of the night don't really seem impulsive.
Just my theories.