No, she is very clearly rational. But she has crossed a moral line and, while she deems it worth the price, it has damaged her. Possibly beyond repair. I feel you are absolutely right that she would and Giles would have made a good fit. Even in this, as Giles has to be thinking of his own experience on the brink, although his motivation was so very different.

I found her drinking at the end to be a metaphor for her whole circumstance. She has accomplished her goal and she now accepts the wreck it has made of her life. I do wonder what the Watcher's Council will do.