In my opinion, there are several motives that could be explored. FWIW, I find it uncomfortable writing details about these possible motives.
As far as the sex ring, I think it is possible DH made it clear to all the sicko's involved, Brittney was off limits. I believe DH may have been infatuated with Brittney. How long had Donnie been contemplating suicide? I believe it is possible DH took advantage of and hurt Brittney that night. Meaning he acted on his infatuation. DH being in a suicidal mindset acted on his fantasy. I believe Brittney would have put up a fight. Ultimately, not ending well. I believe DH knew Brittney would not have kept this quiet. This is all speculation and I really don't like writing about it. In my opinion if this occurred, DH had motive to kill Brittney. What happened next could go many ways. I believe DH may have confided in WWH. However, not sharing the real story. Donnie could have told WWH that he had to kill Brittney because she was going to talk to LE about the rape investigation. In my opinion, this is where a coverup would begin. IMHO, this is one possible motive and one that includes at least one accomplice to help coverup what happened to Brittney.
Investigators stated Brittney was the 'golden nugget' in the rape investigation. If LE knew this, you bet the rest of the sicko's knew the same. Whatever Brittney knew may have been enough to put these scums away for life, rather than a mere few years. In this theory, it is possible DH was not involved. It is possible the other sicko's made the decision Brittney knew too much and conspired to commit murder. Remember WWH commented on DH's behavior and the statement he allegedly made that he was not going to go to prison for something he didn't do. In my opinion, this is another possible motive for murder.
These people are scum. In my opinion, if they could rape children, they could easily commit murder. The scums involved in this sex ring had a lot to lose and if taking someone out could help sustain their selfish sick lives they wouldn't think twice.
I struggle with the motive too. If it was to do with the child sex abuse, either Brittney knew something that no one else did, (or if there were others, they were deemed by DH etc much less likely to speak out about it), or DH etc believed that somehow she would be a catalyst/organiser/support/encourager/? for those who were starting to speak out. The golden nugget comment is interesting, (not a term I'm familiar with), clearly the local LE think that Brittney was a really important, crucial even, part of the chain. I wonder why?
(Surely though, even without Brittney, DH was in so much trouble that there wouldn't have been much "point" to her murder?)
Again to do with the child sex abuse case- which, by the time Brittney disappeared must have been all some of these people were able to think about- if she did go to confront DH, perhaps it just got out of hand, a heated confrontation, perhaps she was so angry that she threatened him, and he struck out/shot/mortally wounded her, "in the heat of the moment".
I must confess, I do give some crediblity to the theory that WH was somehow involved, especially if DH had been infatuated with Brittney, and she thought that Brittney might be about to help the prosecution of DH- eliminate a threat, in two directions?
The LE response to both Brittney's disappearence and the child sex abuse ring seems very slow, and disappointing. I wonder if Brittney's disappearence has caused the local LE to focus their attention on what was going on in that family more? (Isn't it ironic?)
Is it the general concensus that Brittney was not a victim of this group?
Oh, I hope she is alive out there somewhere!