This sounds like the same thing I reported in another thread: https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=885180#Post885180

Just like @The Red Queen says, other "guards" in the vicinity of the Grymforge fight with Nere end up aggroing, and their voicelines make it sound like the heroes are starting trouble, even though (in my case as well) I allied myself with Elder Brithvar.

As a work-around, I ended up reloaded a save from before the battle and making sure my paladin didn't personally finish-off any enemies. Felt somewhat unnatural to fight that way, but at least it didn't trigger a violation for my Oath of the Ancients paladin this time. :-\