It was a surprise when he was charged for Sandra; that murder was widely believed to be by Bittroff. Even LE drew a circle around Bittrolff for that Murder.
A popular content creator and researcher, Josh Zeman, and others, seemed so sure Tanya, formally known as Peaches, was going to be the same suspect as other victims such as Jessica and Valerie, describing them as the bookends in terms of their placement. He and others were certain they had to be related, but in the end, Rex has been credibly charred with killing Jessica and Valerie, while another person has been accused of killing Tanya, and there does not appear to be a reason to doubt that charge, after the shock wears off.
So, it is clear there are other murderers.
Rex does seem somewhat more social than the average serial killer. It does seem like he probably would have shared some crimes, at the very least, with others, if not actual murder. Some witnesses have said that he could not possibly have been undetected in sex work consumption, but knowing someone is illegally buying sex and not saying anything is not a crime as far as I know. So others knowing about frequent sex worker use would not be involved, per se.
By all appearances, he did things like go to swing clubs, strip clubs, parties and bars, and obtained illegal drugs which could be just for victims but seemed a bit social, too...
And, there is a horrific, credible statement from a witness that possibly put others with Rex and a possible victim, Karen, on the night she was possibly murdered.
But of the 7 murders for which he is charged, I don't think there are people theorizing that those murders involved other murderers.
MOO