And then the problem is: If somebody is running a quest, and I for some reason want them kiled (They looted a friend, they tried to loot my castle etc earlier)... And they run the quest using their powers.. It'd make it harder for me to kill them. And similar things

Quest making PCs is half moving the PCs away from the game, making something that is.. half player, half wiz. It's not really possible for them to retain a balanced relation to the rest of the players.

That's why this is, in essence, not a good idea.