Cuz that would ruin a bunch of kool quests that are 100x better than killing an npc for no reason. And there are plenty of npcs to kill
So what?If u wanna do a certain quest, you dont kill the NPC`s..whats hard to get about that?But why if lets say i wanna make a good character, i have to let Riften be infested with all the scum living in it?Or why if i wanna make a evil one, i cant kill whoever i want to kill and so on?
Just takes away from the gaming experience.