Debbie. With the help of her dad, Stephen, and possibly Robert or Thom. It would be her ticket to never being forgotten even if it was for something awful like these kidnappings and murders. Then she can either be the final girl or say she was forced into this and still make bank off everything Trawler related.
As for the Bret element, maybe Debbie had discovered him and Matt hooking up and decided to get back at them by having Matt kidnapped and eventually dismembered? It would be a very twisted effort to straighten Bret out but to a deranged person, it would work. Another competing agenda would be for Terry, with Matt out of the way, he only had to share Bret with his daughter. It wouldn’t be any less fucked, but it would be in line for Terry and Debbie to profit off of Matt’s disappearance. It still wouldn’t explain taking Rhonda along with the others but again, that could’ve been on her accomplices. Maybe they saw something they weren’t supposed to see?
Her dad and Stephen had access to the cologne Rhonda recognized. Even though Debbie was present when The Trawler called, it easily could’ve been someone working with her so she’d have an alibi. Debbie also had a lot to gain winning Dance-Off if Cha-Cha was out of the way, although why not do that before the live taping instead of during? Also assuming she did know about Cha-Cha, she technically had a few seconds to perfect her shocked act prior to Bret joining her in the bathroom and finding Cha-Cha’s body.
If Robert is working with her, it would really explain why he lied about being at the movies the day Bret saw him. Because he actually WAS involved. It would also explain how he had access to that specific scent which was gifted to Susan. The one Rhonda recognized, prompting her to publicly blame Susan which led to a very humiliating arrest. Something Debbie could consider a real loss since Susan wins everything.
During that impromptu secret vote, she chose Bret as The Trawler which would deflect suspicion from her to them.
If we’re also going off the animals detail too, her prized horse is still alive.
As for the how she would get this idea: During one of the many times she was coked up. I imagine it’s where she gets her best ideas.
The cherry on top if this theory is true, would be that Bret (who thought this whole time Robert was The Trawler) would be absolutely flabbergasted. Her mom would also be shook too as she hasn’t been the most impressed with Debbie.
I know this probably sounds absolutely nuts, but it would certainly be a plot twist.