There are at least half a dozen things he could say to jam that metaphorical knife between Xichen's ribs right now, not the least of which was that of course he had left something behind. And keep going from there.

...congratulations, Huisang. You tore through the last shreds of brotherhood and ruined everything so perfectly and completely in one final move. Well done indeed, you little fan-flapping fop.

So he doesn't say any of those things, and lets Xichen get wherever he's trying to get to with this conversation. Obviously he had become something else. Or was there more, more indignities to be suffered in death beyond being marked as some kind of beast. He nods once, placing his other hand over their already clasped ones encouragingly. "Whatever these changes are, they're favoring you at least."