I think I have this figured out, and the reasoning behind the murders, thanks to Legacy's post from yesterday and input from Anne, 2gone, and everyone else that's contributed. Despite that, I still can't tell who pulled the trigger, but I do believe that person's last name started with a B. In fact, the man in Monahans or Pecos's unflagging interest in this case points even more so to a B being the killer, and thus removing anyone else from suspicion.
Mattie was in the wrong place at the wrong time. Patty was eliminated because she was having sex with a B. That B would've been facing charges for statutory rape. If Patty was pregnant that would have been the icing on the cake. It sure would be great to know if this B had had any charges brought against him for anything else prior to the murders as this could have been all the more reason for Patty's elimination.
Patty was eliminated to keep a B from going to prison.
The 3 Bs would have all had a motive, and shared the same motive--keeping one of them out of prison.
If Monahans man helped the Bs, his aid was enlisted, after the fact. Most likely, he wasn't told the truth at the time of why he was being asked to help. The Bs could have called him, told him some man was gunning for their son because the son had slept with the man's wife or any other likely scenario that required that son be ushered out of town in secret and post haste. Shortly after getting involved Monahans man would've learned the truth, and he would've been hopping mad because the Bs had placed his standing in his own community at risk for having been involved at all.
Monahans man may have had other friends in Eddy County that would've been sympathetic to his plight.
To me, this is the simpliest explanation, and the most logical scenario.