One is murdering humans in mass, the other is not. It's incredible to see "both sides are in the wrong" when one side explicitely said that they are murdering the mortals.
It's incredible to see you claiming that one side
isn't in the wrong when that side broke the Oaths that kept the spirits from slaughtering mortals
in the first place. Had those oaths been kept to, there would have been
no civil war, no massacres, none of this. But that side wasn't the least bit concerned with that, now were they? They just acted without considering the consequences, believing that daddy dearest would protect them and that
of course one of their own would be chosen to be the next King.
Because let's be honest here, any White is more like a Spirit than a mortal, and with the Weilu that is only amplified. The only thing holding the previous King back was his love for his sons; there was no such block for this King.
Life isn't so clearly cut into black and white, right and wrong. Is one side
more wrong than the other? From our standpoint - and even the standpoint of the 'modern' Imperial Cultivator - absolutely, but that doesn't mean that both sides aren't
wrong. From the standpoint of the time? Kill the traitors, and everything associated with them. I'm certain that just about
every powerful Cultivator clan of the era would agree with that. Most modern cultivator clans would, too, though they would probably manage to limit the damage to just the family in question, and not the mortals under their care (except as unfortunate collateral damage).