Fascinating find!
If you attack "nonhostile" Goblins, who just raided some civilist, and are roasting Dwarf (wich my Paladin is aswell) as you are speaking ... you break your Oath.
But!
If you side with Karlach, slain all those false Paladins, then come back and kill her aswell ... all good.
To have non-hostile Goblins there, you need to have pacified them before entering their territory and you break your previous decision/behavior by attacking them afterwards despite them not being aggressive toward you. Not a very honest or merciful of you. And I had the impression they changed the roast to not be a Dwarf in a previous patch?
For the Karlach vs Anders, it depends how you do it, what your character knows in-game and the conversation you had with both side before killing either. I've seen someone complain he got the Knight visit after killing Anders on Reddit.
Just like it's possible to oathbreak when dealing with everyone favorite Auntie Ethel. It's all about what choices you make and how you behavior afterward. Not just killing X or Y.
Also, currently the game default to a Paladin of Lolth.
