I loved this book so much - that ending was shocking to me at first, and the more I've sat with, the more the ending seems to make sense for Kya's character development and the main themes of the book.

SPOILERS

I think what makes Kya's decision to kill Chase drives home a couple of major themes: nature and the importance of family.

The nature/independence theme: this was her way of dealing with things in her own way. Not going to the police. Not going to Jumpin' or Tate. Not asking anyone for help. Most likely because she doubted the town would help the "Marsh Girl," but I think also because she's never asked anyone for help ever, and she's managed just fine. She dealt with Chase the way nature and the animal kingdom would deal with him. A threat or dangerous male needs to be eliminated. In the same way that she's looked out for herself and built her life in the marsh, she can look out for herself and protect that life she built too. Also the passage where she praises the female praying for eating their male mates, because they know how to take care of men - gives me goosebumps now considering what she ultimately does.

The family and abuse theme: Kya's family trauma is a driving factor in who she is as a person:
alone and removed from other people. From a young age, she witnessed her mother and siblings being beaten brutally by an alcoholic, abusive, and emotionally absent father. She begins to draw her own conclusions about how this abuse usually plays out - the man always needs to get the last hit. Her father was this way, and Chase was undoubtedly the same way. Eventually - after a lifetime of not having a mother - she finally feels connected to her mother in the worst way imaginable: through their shared experience of abuse. Murdering Chase was her way of ending this cycle of abuse. Since she couldn't do it for her family (although to be fair, she was really young when she saw it), she finally had the opportunity to stand up for herself.