If illegal drugs have a part in this (I think they may), then it would likely be far more involved than just SP killing MIssy because she found out he used drugs. In my scenario, it's because that knowledge leads to new knowledge about who supplied him with those drugs...and how important that person was at the local level (trusted community leader?) and rampant such a drug operation might be. A large-scale corruption involving community leaders and/or dirty cops - one that possibly was county-wide or even involving multiple counties- would be a scenario for a hush-up hit, IMO.
Purely hypothetical scenario:
What if MB 'discovered' drug use by a minor she knew, maybe even ran into frequently at the gym. What if she knew minor well enough in that small town that she approaced him privately about what she knew, about her concerns about his drug use. What if she promised not to tell his folks or the authorities on the condition that he stop using (eg. if she saw or was aware of transactions at the gym). What if minor and/or suppliers knew she was deeply bothered by this recent discovery (possibly because she discovered after talking to minor that he wasn't the only minor using, that trusted officials were involved and supplying, endorsing, or at least agreeing to look the other way, etc.). What if this was why she'd seemed more 'quiet' lately and had stopped attending the gym.
This then becomes a community corruption scenario...not just a single person using drugs. This 'community corruption' scenario is what I personally theorize was the motive in all this (given my observations concerning the recent Ellis County news stories and otherwise). Not just drug use by a single person, but something that, when brought to light, would snowball enormously.