In Morrowind you could kill ANYBODY. But if they were essential to the story it would warn you that you had broken the game.
In Oblivion it was applied to a very small number of absolutely essential people without whom the main quest could not advance.
Then by Skyrim it was pretty much EVERYONE who featured in ANY quest.
Now it's ANYONE WITH A FREAKIN NAME.
It would be nice if the devs thought a little harder about what the word "essential" means before checking that box. Fallout 2 could be completed even if you killed everyone in the game - or no-one at all.