Essentials in Bethesda games (especially FO3) feel like they were assigned by a bunch of different people.
At the beginning everyone was killable, except for children. Three Dog can be killed and is replaced by another DJ, you've got different responses if for example you've killed targets in You gotta shoot 'em in the head before starting the quest, you can kill Moira before even starting her quest and there is a dialogue between Burke and Roy if you decided to blow up Megaton after helping ghouls take TT.
Then Emil added tags for plot character and set pieces (I can't call Janice Kaplinsky a character when her whole role is getting shot by Autumn). You can't ever kill James, Dr. Li, Janice or Owen and Sarah Lyons.
Then someone in sidequest and DLC design got lazy and added essential tags to Zimmer and his bodyguard, Desmond (Point Lookout), Pinkerton, Midea and Werhner (Pitt) and guy that gives PA training, until they are no longer useful
Then someone drunk got his hands on CK and started giving tags left and right. Harkness, Abraham Washington, Daniel Littlehorn, Regulators' lady, that unnamed guy at GNR, Rivet City doctor, Rivet City guns vendor, Citadel bouncer and unnamed guard are essential forever.
So at the end you've got some plot characters that are essential to "story", characters that are essential until you complete their sidequests and characters that are essential because.
You also have plot characters that can die and there are some minor changes to the game (Overseer, Three Dog), characters with massive side quests that can be killed without a hitch (Moira, Reilly, Paradise Falls slavers), traders that die from broken spawns, quest givers that die from broken spawns (Arefu, Andale, Big Town, Temple of the Union and Lincoln's Monument), companions that can be killed pretty easily by broken spawns (except for Fawkes, because he has a billion HPs) and a character that breaks a game if he dies (Paladin from Lyons' Pride is tied to Liberty Prime's movement in Take it back!)
At least they fixed this issue in Fallout 76...