They simply remove all nuisance in an attempt to assasinate Serie, For Land and Ubel it's quite obvious they work for Serie, as for Frieren they simply see them as nuisance as Frieren cast tracking magic to track them, so ofc they would see Frieren as threat/nuisance.
Okay but why would they want to kill serie again i forgot
i think they don't specifically tell the reason yet, same with Frieren being targeted by assassin not too long ago. But if i may guess the reason because the way how Serie acts for her pure interest only ? They might fear she might be a threat next because of her power and that's also the reason why they targetted Frieren too back then.
What is the reason for them trying to assassinate them again? Cause this all seems pointless