So it is a power struggle. There is still some missing points about the real motivation of the shadow guys and what they want to accomplish. Imperial mages are not necessarily on their side, from my understanding of the dialogue.
But the disrespect to the hero party is still odd (the statues were the first hints). Maybe the power balance was disrupted when the demon king was killed, so there might be some kind of resentment because some people were benefiting from the demon king era and now want to regain it.
I understand for Serie, but it is a bit sad to target Frieren, since she has zero incentive to cause any harm to any power structure. She just want to discover folklore and enjoy bits of life.
There is also Denken. He is (was ?) an imperial mage and is now a first class mage under Serie. He was helped to save his hometown and gain a really powerful spell. I still wonder what would be his allegiance in pratice because we don't really know where the imperial mages goes in the power structure. But I find it hard for him to go against Serie and Frieren.
Last point, I don't think Serie would be killed, it does not really match the manga style from my point of view. She probably has the power to destroy a whole region if she feels to, though. But let's see.