Yup kill everyone, then complain that none of the quests are working.
Message pop up? Then you complain about a message popping up everytime you kill an important NPC. Then having to reload the game.
Respawning NPCs? Then you complain about NPCs coming back to life after you kill them.
I would prefer respawning NPCs. At least they can't act as witnesses. I mean, after murdering all the witnesses only to find that this little bugger here or that little brat of a child has witnessed your crime and CANNOT BE KILLED...:rant:
At least treat them as dead for awhile, or make them lose their aggro on you.