I am making no excuses for either one of them......they both disgust me. But it makes more sense to me that TM murdered Heather and spineless SM is covering for her. Maybe she did believe she was going to lose 'her man' to a beautiful younger woman who looked much better than she did, imo.
She seems to be the master manipulator and controller, imo.
IMO
I do feel that TM is the one who harmed Heather. I feel she tracked SM the night he went to meet with Heather, and based on TM's self-professed love of guns, think she might have used a gun when she came upon SM and Heather in SM's vehicle, to demand that SM and HE do as she said that night.
IMO, she probably had a loss that hurt her deeply earlier in life (i.e. divorce of parents, etc.), and it seems she to me that she tried to create the perfect Disney fantasy life for her children that she wished she had had as a child with her Disney themed home, Disney themed RV, and Disney vacations.
IMO, she home schooled them so she could keep them "safe" from the outside cruel world. I also believe she tried to be a good wife as well as a good mother.
When SM strayed, I believe she felt that the perfect world she created was being invaded by the evil outside world, and I believe Heather represented much more to her than just the "other woman", but the evil and destruction she was trying to protect her children from.
I also believe she had other age-related self esteem issues that compounded the problem, but I believe her fantasy world being invaded by Heather (in TM's mind) made the cheating incident that much more than what it was.
I also believe that her recent love fest on fb with SM was her trying to return to the delusional perfect fantasy world she had been formerly trying to inhabit.
ETA If Heather was in fact pregnant, that would have made her even more of a threat to her "perfect" fantasy world.