River

okay here’s why because this has become a theme again:

Cotton and Maureen WERE having an affair, this was known as true by the end of the movie.

So the plan was wait for Cotton and Maureen to stop doin’ it, then go inside and kill Maureen. Billy and Stu (and by extension Roman) planned it this way so that Cottons DNA would be all over the crime scene. Police would take it as an open and shut case.

Why Billy and Stu COULDN’T have raped Maureen:

Billy and Stu would’ve left their own DNA both on and around Maureen if they raped her, meaning they would get detected and jailed for the crime.

We know Cotton was there right before it happened, so it makes complete sense him and Maureen had just had sex THEN Billy and Stu went inside.

Guys it’s all about the DNA and leaving mass amounts of evidence there. Billy and Stu would’ve been caught if they raped Maureen, especially inside her own household where she was found IMMEDIATELY after the job was done (found from the line “she watched her mom get butchered” in the bathroom scene) Billy and Stu would not have had time to clean anything up after leaving.