Both my husband and I had terrible MILs. I often joked that’s why we work so well together- nobody else could understand the crazy we came from.
I think part of the problem for some MIL is a societal/generational thing.
A generation or two ago, the mindset that elders are to be respected no matter what, put family first at all costs, was very prevalent. Must play Happy Families no matter what. And, there was the refrain, “When you’re the elder, it’ll be your turn.” and, “How do you want your kids to treat you when you’re older? Set the example.”
It resulted in generations of women who had little power over their own lives, dedicating themselves to their children, their husbands, and following the lead of their mothers, MILs, grandparents, etc.
Then they became the MIL/grandmother. It was finally their turn!
… except it wasn’t. Because thinking has changed. Nobody wants to do as they’re told with their marriage and children. They want to make their own choices, toss out traditions they don’t like, and have boundaries.
So there are some older women who are angry, confused and grieving. They did what they were told, and were promised their turn would come. But it hasn’t. Kinda like working for decades toward a promotion, only to have the position eliminated at the last second.
Unfortunately, rather than getting therapy to help process their feelings, some of the women who experienced this try to force compliance, blame their CIL when they don’t get what they want, and take their anger and resentment out on those around them.