this bit is the scariest bit of the entire episode
in that last gif, Castle’s eyes change. He becomes his protective self, the one we saw in Target/Hunt. It’s almost like he makes the decision, or realization, right there that he would die for Beckett and that might be a very high possibility.
I think there is a little of that he’d do anything for her there.
But also, I think he sees too that this is the sort of thing that could make her slide into darkness again, chasing after dragons. And so, immediately after this, he gets up, and draws her away.