That was a brilliant conclusion to the story, I loved seeing Castle in a different light, it was even more intense than when he went looking for Alexis in Europe. Even though I knew it wasn't Beckett shot to bits in the chair the way they played that scene got to me as Castle reacted so distraught and you felt for him regardless of your mind telling you it's not her.
I thought the way Castle went on the offensive, used his writing brain and played a clever cat & mouse reverse with 3XK was classic and so well executed, especially the final dialogue between the two before the shot and then the cut to Espo with the smoking sniper rifle.
I don't think Beckett has ever had so little dialogue and so few scenes in an ep before but all of them were tense and chilling, I thought the way they didn't actually show her killing Nieman but instead we simply saw her grab her and then from behind with the blood dripping off her hand and the scalpel was really effective, less is more. Annie Wersching was really cold and creepy as Nieman, a very different role for her.
I think the fact Beckett didn't go rushing to Castle but simply rested against him when they reunited was another good choice, and the fact the whole thing had affected her as opposed to some jolly ending or her swaggering out like a badass made it all the more impactful, the final scene with her and Castle illustrates why they are the best TV couple in a LONG time in my view, they feel genuine and the way they talked was like real people, not overly wordy character types..
This is my favourite show on TV and this may have been it's fienst hour to date!