If you kill that NPC and you never got the quest, how would you complete the task? If you can't complete all these tasks how do you gain XP and beat the game?
In a tabletop RPG players often comes with their own ideas of a guest like "lets burn a pub and rape a waitress". Good dungeon master then, prepares ad-hoc quest.
Computer game could use similar technique. As soon as you kill couple citizens (and do not receive any xp) a new quest is self initiated - new journal event : "Lets purge this city, to the ground. Everyone seems infected by the source. " when you finish it, you get your reward.
e.g. Not every quest must have a 3rd person of quest quiver.
or another one
as soon as you pick pocket few citizens a message boy popup with a letter from local chief of underground asking for a favour. Pickpocket certain people.
e.g. get noticed, you could even have a person the quest quiver who is not visible or players could meet. For example a mage who communicate thru crystal. Or a thug chief who is somewhere underground and you could only initiate a dialog with a sewer bar on the ground.
Well, I am not a big fan of traditional quest givers, standing in the local pub all day long waiting for a band of heroes. Most of quests should be hidden or initiated by some action.
The main quest line could be self powered - e.g. main characters have an idea what to do. So there is no big boss who pays them, they fallow ther own will. Would want to save a kidnapped princess and kill the wizard, maby they would rather offer their bodyguard services to the wizard and rape a princess as reward. Depens on good/evil character. Anyway, finaly they will live happily ever after on the wizard's castle with a princess.
