The ending was unsatisfactory to me only because in The Expats and The Paris Diversion, Kate spends A LOT of time criticizing Dexter, basically calling Dexter a liar, and then, ultimately, rescuing his sorry self from serious consequences. Why run all over Paris, send your husband to a safe house, use your secret assets, risk getting shot, and risking her cover only to turn over what the thieves were after all along? But I don't think Kate's choice at the end of the book was necessarily about forgiving Susanna. Kate had a terrible last mission in The Expats, one that caused her to leave the field forever. I think some of what was going on at the end of this story hit too close to home. Maybe Kate decided to finally forgive herself. By forgiving herself, she needed to stop.
Revenge was most definitely the driving force. Kate and Dexter had something that Susanna and Chris wanted. And it was something they thought they were owed. I'm amazed by how complex their plan was and how many innocent people they were willing to hurt to carry it out. There is the injustice. Also, are you really righting a wrong if the action that set all this in motion was illegal in the first place???