I'm not sure if it's accurate, but I felt like last night wast the coldest she's ever been.
I think it's actually the opposite. It's devastating but not cold. The Elizabeth from a season or two ago would have done a much better job at keeping an emotional distance and doing what needed to be done without remorse. She would have been playing Young-Hee rather than actually befriending her. That she tried so hard to find another way, and showed such shame and actual sorrow for her actions at the end is a marked change, and that she does it while feeling and showing such emotions rather than doing so coldly is what makes it so much more affecting.