It's more having the option of doing it, rather than an itching to do it. Some people are going to want to kill certain NPCs, and it'd be bad to be prevented from doing so just because the game says they're important, even if you don't care. I don't know who my character(s) are going to like or dislike, and if one of them will want to take someone out, I don't want a little crown icon to tell me I can't take a specific NPC out.