+1 Exactly this.
They should also tag the NPC's as belonging to certain quests/peaceful dialogue resolutions - so if you do the peaceful way out and get XP for that, you shouldn't be able to murder them afterwards for double XP.
I would love a confirmation from Larian that XP from quests/dialogue is in the works so we can stop reiterating on the same ideas constantly. Do they have a list of known/accepted changes, bugs, requests?