While I get where a lot of the complaints are coming from, I was actually alright with the ending. To prevent from spoiling I won't go into detail, but while it lacked deescalation, it was still powerful closure and elicited such a strong emotional reaction from me that I couldn't stop thinking about it long after I finished reading. Our female lead is forced to make such a difficult decision that as a reader, you just can't help but wonder what would've happened had she chosen differently.
Some other complaints were about the love triangle. I personally found it surprisingly well-executed, mostly thanks to Goodman's decision to take it beyond simple love—the relationship between Eona and her love interests also deeply explore the ideas of morality, power, ambition, and trust.
(Also, I'd recommend reading this book as an older teen or young adult. I read it for the first time when I was still in elementary school, and there were a lot of themes I failed to pick up the first time around, simply because I was too young to really understand them.)