Unless you double dip by killing after the quest, it isn't always more XP to kill instead of resolving things peacefully. I can think of a few examples, if you're interested. Perhaps it is more rewarding more often, do you have any examples of your claim?
Generally speaking once you have resolved situation, either through violence or other means, you expect the player to move on. This creates two or more "core" methods of resolving each circumstance. For an extreme example, it would be unusual for a person to talk someone down from committing suicide only to follow that action by killing them. The idea of "double dipping" as you have mentioned is a real thing and currently very possible, but you can't balance exp around that possibility because most players are not going to do it.
We actually discussed this idea earlier where I suggested removing exp from killing enemies after you resolved a situation peacefully and other people don't like that solution for their own reasons. I don't really care whether it gives exp or not because I will choose not to do it. If other people do, for whatever reason, good for them.
Originally Posted by error3
Players who do more will always have more XP, but it'll mostly just be an extra level at any given time, it likely isn't too impactful. This is the case for people who kill all optionals, do all sidequests, etc. Even if the rewards for good/evil were exactly the same (which would make the choice super meaningless).
We are talking about two solutions to the same problem, there is no doing more or doing less in this situation. There have already been a few examples of where you get less experience for choosing the peaceful option. I suggest reading up a ways. I would be curious to hear your examples of when the opposite is true unless of course the only example is taking the peaceful option and then turning around and murdering them. I don't care about that option.
