Seems like the conversation has changed a little since the last time I posted so I will give my two cents as well on the subject of killing to gain xp after the quest completes.
I am a long time PnP roleplaying game player and the systems for PnP pretty much address and take care of all of these issues. If larian should so choose to keep a player from taking advantage of the game system as it stands now which it seems like people are by killing quest givers after completing the quests (shame on you people for even doing it) Then the devs should set a flag that makes the npc give them 0 experience for assassinating them or if they are lower level than the npc 1/2 experience for killing them in an actual fight. Most PnP games have this sort of methodology. However if someone is evil or kills a vendor for any reason they should get all of the loot including skill books, gold, and items that the vendor had when the player killed them and only give 1/2 experience if they get to fight the character or 0 experience if the character assassinated them.
The main issue seems to be that people are thinking that experience is to be maximized this is in fact what is called meta gaming using the rules of a system to min/max your character is meta gaming. Using your knowledge of a system in order to break it is meta gaming. Forcing a game developer to put rules into a system they should not have to worry about is caused by meta gaming. If you were the character would you really stab the person after completing the quest and having the reward you? After all they might higher you for another job later on.
