UrsusArctos: The script (translated by Reichu) rather enigmatically calls this a "smile of betrayal". The "Casablanca Theory" mentioned by Wayne in Cut 278 is one of the few sensible explanations I've heard about this: that Kaworu caused this entire great uproar to draw Rei to Terminal Dogma and tell her the truth - that she's the vessel for Lilith's soul, and her old body is the one on the cross. Is he smiling because he has betrayed the truth to Rei, or is it because he's betraying Rei by asking Shinji to kill him?
