Err, looking at the issue of xp granted and exploiting that, I actually don't see much issue? Dnd 5e has a max level of 20, and true you could try to min max experience gains by questing then murder hoboing BUT the game will still penalize you likely. People will react very poorly to that and if the saw the murder, go directly hostile. It is also likely you could be cutting off future quests by killing them, or creating a bad reputation where certain factions will attack on sight. So you need to balance your wants of xp vs other game rewards and interactions I think?
