Honestly, this is a happy ending for Si-ae. She likes sex, and doesn't really want a normal monogamous relationship. That's fine. For her, the two men in her life are opposing but equally toxic relationships. Gi-jin stripped her of her power while her husband stripped her of her sense of self. Her stabbing Gi-jin after having sex with him was facing her addiction and taking back her power by saying she didn't want him in her life after getting another hit. Her leaving her husband was acknowledging that she tried to give up her sexuality and desires to conform to normal society. In the end, she goes back to "thotting" but she picks the men, she initiates, and she does what she wants. She's no longer ashamed of what she wants nor is she concerned about what society thinks of her for not conforming.
Honestly, this is a happy ending for Si-ae. She likes sex, and doesn't really want a normal monogamous relationship. That's fine. For her, the two men in her life are opposing but equally toxic relationships. Gi-jin stripped her of her power while her husband stripped her of her sense of self. Her stabbing Gi-jin after having sex with him was facing her addiction and taking back her power by saying she didn't want him in her life after getting another hit. Her leaving her husband was acknowledging that she tried to give up her sexuality and desires to conform to normal society. In the end, she goes back to "thotting" but she picks the men, she initiates, and she does what she wants. She's no longer ashamed of what she wants nor is she concerned about what society thinks of her for not conforming.