This is an interesting question, and I invite readers to chime in as well with their own suggestions for the listener. I think the most important thing to do when you have a situation like this and are finding it hard to judge is to go back an re-read the Experience Points section in Chapter 7 of the Dungeon Master Guide.
So lets look at the situations, and see how the DMG rules it. The listener gave quest xp for finishing some sort of quest, and the player tried to kill the npc involved for his xp value. The first thing I’d ask is “was that an encounter?”. If the answer is no, and the pc’s were in no threat, then no xp should be given. Remember, the pc’s can’t walk around with a bag of rats to kill them and get xp. If there was no challenge, no encounter, no threat, then there should be no xp awarded.
Remember that xp is a reward for overcoming challenges, not a payment for killing every living thing in the game.
Now, your idea of altering the way you hand out xp is perfectly valid, and the DMG presents several ways to do it. In my own game, I hand out XP at my discretion, usually at the end of the night, and I monitor the rate I advance my players. They aren’t counting every experience point they earn or hope to earn, and have learned to assume that I’m advancing them as they should be. It works out well for us.
In the end, the best advice I can give you is use your DM’s power at the table. You’re in charge of how you run your game. Use your discretion, common sense, and be fair. If a player is clearly taking advantage of a situation you’ve laid out that wasn’t meant to evolve into a combat, judge it as so. “That’s not a challenge, sorry, no xp, and now you’ve needlessly killed a member of the town guard. Oh-oh.” Turn the situation into a complication. Maybe that will get them to see that their actions will have consequences for the entire party, and they’ll think twice about doing certain things.
I hope this helped a bit, and I encourage readers to give their own opinions in the comment section below.
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