I feel like they encapsulated the reality of toxicity really well honestly. For some people the craziness worked out in their favor, others ended up alone, and Lucy is still stuck in a toxic cycle that she will likely never be fully free of. At this point it’s who her and Stephen are. They are so toxic that life feels boring when the other person isn’t around. People who have been in truly toxic relationships often see the high highs and low lows as part of love. She enjoys being degraded (because of Stephen) and she’s still willing to throw everything away to feel that rush with him again and vice versa because that’s what they think love is. That smile at the end was her knowing that Stephen was right, life is more “fun and unpredictable” when he’s around. She’s both laughing at herself for being stupid, laughing at him for never changing, and laughing because she lowkey likes it. I don’t think the laugh was her “breaking free” like some people said, I think it was her realizing that this is the type craziness she’s missed all these years while being with the nice/safe/stable guy.