When he makes it back into the room, By is kneeling over the ghost, hands wrapped around its throat, pale-faced with rage. That cry of his name went unheard, or came too late - because, for all that Bastien had just barely been out of sight for a moment, some dilation of time or some irrationality had put the thought in Byerly's head: I killed him. It had felt, with the full weight of truth, that he had pushed Bastien fatally down those stairs.

And so: the rage and grief has taken him; he is lost to the emotion as he throttles the life out of his cousin. There's a hard certainty to his actions, that trained instinct that he lacks in times of lesser passion; the fury is so great that there's little of Byerly in it, and more of just the trained spy who had been considered, once, for the role of assassin before being discarded as unsuitable.

He doesn't see Bastien reappear. Perhaps his eyes are clouded by the magic. Or perhaps it really is just the hatred.