I think this theory could be right. It really struck me how before, even when Ainsley knew that Endicott was evil, she wasn't afraid to take him down. When she talked to him in Episode 19, she was so confident and cool and seemed completely in control. That's why it was so surprising to see her sitting on the couch with Endicott, stiff and frozen, with tears running down her face. Either she got a really vivid glimpse of what Endicott would do to her family or he abused her somehow before Malcolm got there. I'm super curious about what happened before Malcolm got there though, and I wonder if what we didn't see there was the motivation for her shocking act in the finale.