Swallow's use of that plot device was necessary if he was to portray Valeris as a sympathetic character. Perhaps it would have been more "interesting" if she was a coldly calculating menace, but that also has been done in other novels.
I didn't walk away from Cast No Shadow
with any inkling of Valeris as a sympathetic character, and while I am not the author, I can't see how the idea of making her sympathetic ever entered into his mind. (He is free to correct me if I am wrong.) The conclusion of her story in the book makes the fact that she isn't a sympathetic character crystal clear.
I have a somewhat opposite view. Star Trek VI positioned her as a traitor and a murderer - without giving any context to her decision. This book seems to be designed to provide that context, flesh out her character, explain her thought process, and do so in an entertaining framework. It also highlights the motivations and actions of various types of "traitors".
As to her final action: