I play a lot of DnD and Pathfinder and this is an extremely common issue in those worlds which, in my opinion, is best handled by giving the same amount of experience no matter what happens. It doesn't matter if you decide to take the direct route and mow everyone down, talk everyone down, or talk everyone down and then jump them while their guard is down, at the end of the day you have "conquered a challenge" and should be rewarded depending on the difficulty of the encounter. The one way that exp should be variable is if there are optional side objectives, stated or otherwise, that alter the difficulty of the encounter or create unique ways to handle the problem. In almost all cases creativity in how to handle a situation should at least be attempted to be rewarded but that can be pretty hard to do without making it feel shoehorned or not comprehensive.
If killing everyone gets you 100 xp and doing the quest and turning it in without killing gets you 100 xp and turning in the quest and killing everyone after gets you 100 xp. Why in gods name would anyone go through the extra time and effort to do the fights?
Do you see the issue?
On the other hand, why do you care if some one puts in that extra time and effort and ends up overleveling?
Going the "good" way is only gonna put you at the correct level...probably higher, anyway, if you do the optional stuff.
And at the end of the game the level difference will be....maybe a couple levels. Is two-three levels between a casual player putting in 40 hours in a "good" play through and someone meta power gaming as "evil" to be slightly above you at 60-80 hours at the end worth crying over?
I have yet to see someone actually make a case against that.
Hell, the most I've seen said is that it's too easy to go with the kill option cause it simply requires a little extra time and no real effort.
My response was Bovine Defense Initiative to make killers work for their extra ill gained rewards and karma system attached to reputation (so it actually matters) so one actually thinks before killing. Also, specific quests tied to spruce action sequences/reputation.....
Edit:
I power gamed in D&D all the time or advised other s how.DM response? Ramp up difficulty too match and/or have specific ivy triggered events based off character actions that would legitimately attempt to punish me by making me prove I "deserved" my gains.
This took place in the form of bounties, groups of city guards, pissing off elementals, etc...They didn't have to change game system rules on me; they used creative solutions that actually made sense and added to the fun without making me frustrated by making me feel like an impotent limp rag with only the barest illusion of real choice
Generally, this meant different quests, box interactions, difficult in over alll combat, and different gear and loot. "Good" players would have a much easier time gaining blessings, high level loot rewards, etc...they'd be fewer but more noteworthy for all the work and then there's the inevitable unique story it creates. "Evil" char had higher levels and generally more loot with a better average level, but the difficulty in gaining really high powered stuff was very risky.
Ex. Someone pure calling to angels for help has a much better chance at getting what they want vs someone calling demons who might just kill them for lolz....unless the summoner is just that skilled and well prepared to attempt that fight and even then it was really damn hard
