Watched it purely because of Day-Lewis, since it's supposedly his last. The movie progressed quite predictably for the first part, but ended in a place I totally didn't expect.
All I can say is that's a very screwed up relationship. And she's bold in ways that are very dysfunctional. All I could think of is what if she got the amount wrong or just got tired and decided to actually kill him?
I guess he could treat it as a cleanse to refocus? I don't know... Just very screwed up, those two.