"He just literally destroyed an entire kingdom not too long ago."
Nah, bro, I don't think a single castle was an entire kingdom. It's just a single castle, with probably at most a few hundred people inside, since it wasn't under siege or anything. In fact since it was in the middle of the night, there were probably less people in than during daytime, that is, very few outsiders. The only potentially innocent people would have been prisoners. There wouldn't have been foreign ambassadors present, nor any petitioners. Just the king's employees, who deserved what they got by being willingly employed by a tyrant. Let's not forget the castle staff was happily helping the king to kidnap and deal with Reene.