Just an out-there kind of thought. And jmo.
I think the killer is someone known and fairly close to the family, maybe even within the extended family (which means Missy knew them, too), someone who perhaps thought s\he was doing Missy's immediate family a favor. Angry at Missy for a reason concocted in her own head. And I feel BB has suspicions on who this person
BB finds\is given\somehow discovers a shirt that had been bloodied, thinks he knows who the shirt belongs to, and gets it to the cleaners along with shirts that were truly from the dog fight (via RB). Because he needs the shirt tested in order to have suspicions confirmed or denied in a seemingly innocent way, without he himself pointing fingers.
I think he may have a good idea who Missy's killer is, but he doesn't want it to be that person (because they are related? Involved another way?), and needs the accusation to come from someone else, preferably LE.
Again, jmo.