Sue Anne's Inevitably Tragic Backstory
Very few horror movie villains are demented and evil simply for the sake of being demented and evil, right? It's usually about getting revenge for some tragic event in their past, and in Ma's case, it involves a group of classmates who still live in town and now have high-school-age kids of their own (two of whom, Andy and Maggie, are unlucky enough to be Ma's original drinking buddies).
As we come to find out, Andy's father, cocky jock Ben (Luke Evans), once convinced a shy, quiet, teenage Sue Ann that he had a thing for her. He invited her out to parties, flirted with her, and eventually asked her to meet him in the janitor's closet at school to hook up. Since it was pitch black in the closet, Sue Ann didn't realize until it was too late and they'd walked back into the well-lit hallway that she had been tricked into hooking up with a different guy entirely. Ben and a large group of students were waiting outside the closet to cruelly point and laugh as she ran away.
Sue Anne was never able to overcome that event in her childhood. Even though she went on to get married (and divorced) and have a daughter, her emotional development never progressed beyond that embarrassment and trauma, leading her to become a twisted, vengeful version of the innocent girl she once was. Now, is that an excuse to sew a girl's lips together and start bashing people over the head with irons? Um, no. Ever heard of therapy, Sue Anne? Maybe a kickboxing class to get out that aggression?