We don't know for sure that was her last POV in the series. We assume she's dead, sure. She may not be, but it's likely she is. Also, it's entirely possible with the overlapping chronology of some of the books that we might still see her perspective before being balefired. It took "Crossroads of Twilight" half the book to catch up to the cleansing at the end of Winter's Heart, after all.

I don't think we're limited to the killer's own POV being the only method of revelation, and even if it were so, that may not have been Graendal's final appearance.