Experience for just killing NPCs should probably be gone completely.
I can understand giving it out for fights that are mandatory or unrelated to quests, but if a quest can be completed without killing anything, doing so should yield the same amount of experience.
A more elaborate alternative would be to have some unique loot attached to the non-violent solutions(or doing the opposite, where it makes sense) and giving out more general loot/xp for the other solution, but the way loot works in Divinity isn't particularly conducive to that approach.
