Does this mean that the empire has been infiltrated by the demons and are being used to eliminate living legends like Frieren and Serie? Or they just think that they're threats to the Empire's reign even though they couldn't care less about political power struggles? What the hell are they thinking when there are still some remnants of the Demon Army running around? Although I guess the Mage's Guild is an organisation that can be a big threat in the future and killing Serie could set back their influence and growth.
I think the main enemies in this arc is going to be humans and politics, at least the chapters and events running up to this arc points that way. But we'll just have to wait and see I guess.
Why Frieren was on the assassination list when she isn't related to the magic association is a bit weird though, since I don't think their relation of Frieren being Serie's apprentice's apprentice is known to the public.
We just have to wait and see how it plays out I guess. But if their enemies are humans, I guess that's also one of the reasons Übel was the first choice of the first class mages.