A story that works on some interesting themes, but weaker than usual [for this author] storytelling weakens it.
I believe the main themes are how power tends to change us, and how relationships can turn abusive. My problem with the storytelling is that he uses a sledgehammer to make his point, resulting in inconsistent characters, and a difficulty in suspending disbelief.
The suddenness of Lisa's transformation does not ring true, particularly as she oscillates between Nazi bitch from hell and her old sweet disposition, almost on a dime, and without much rhyme or reason.
We aren't given any explanation for her very sudden change in behaviour, nor for why she inconsistently reverts to her old caring self at times.
I think the story would have been much stronger if her change into someone unlikeable had been more gradual, and less extreme. For her to be so physically violent with Bert is overkill; psychological abusiveness would have sufficed to support the author's theme, and would have made Bert's staying with her more plausible. In general, abusive relationships evolve into being, making it all the harder for the victim to get out. Here, the change is sudden, and quite physically violent. That Bert doesn't see the danger he's in doesn't speak for his intelligence. Again, if the transformation had been more gradual and less extreme, we could believe his attempts to make excuses for her.
I think this story has potential, but the author needs to take less shortcuts, and use more subtlety to make his point. We need insight into what's motivating Lisa's changes. On the outside, she appears completely bipolar. What's going on for her, and what internal justification does she have for her complete change in personality? Did she have a reason to resent being 'weak' before? And why does this charming young woman turn into an evil bully just because she gains some strength? While the story doesn't have to answer those questions, doing so would make it much more interesting.