If the shadow warriors plan to eradicate the magic (mana) in that world, the demons wouldn’t let it slide, considering they detected frieren going into the past such as this action wouldn’t let it slide. I think I agree on this theory that the mastermind are demons.
I don't think the demons know about not care much about human politics. With the exception of Macht, they probably can only get a rather shallow read on human motives. The magis association nor the empire knows what the shadow warriors goal is, how would the demons know? Esp since the demons aren't a organized faction anymore
I feel like the whole thing with the Shadow Warriors is somehow connected to the Demon King and that demon who could see the future. Maybe they took control of the remaining Shadow Warriors while posing as humans, which could explain why the Warriors have so many powerful/important people on their hit list.
I mean, why on earth would a human want to kill the mage from the Hero's Party who saved the world? Unless it's somehow tied to the demons wanting to eradicate anyone powerful, giving them the edge over humanity.
There's also the possibility of it all being connected to some sort of extreme anti-magic ideology, but I feel like the demon theory would make more sense, as they've been the enemy since day one.