Ren doesn't say anything. He doesn't ask Magpie what the hell made you think threatening him like that was a good idea?, because he already knows. It's nothing, because he wasn't thinking when he did it. It was everything about Crow suffering, because they'd both defend Crow to the ends of the Earth and then some.

He just looks on, impartial. Like a parent who's about to give you a "not mad, just disappointed" speech.