I'm relatively new to the whole Elder Scrolls thing, with Skyrim being my first game. I say this to apologise for not being great with remembering names and stuff.
I was on a quest where you have to make a sacrifice, preferably a "non-essential" follower. I wasn't sure who is essential and who isn't. I had J'zargo following me at the time and thought oh well, and gave him up. Reading this thread, I am guessing that because I was able to kill him he wasn't "essential" to another quest??? I pray he isn't, at level 43 I'm too far in to start again now!
I don't know if it is a bug or not, but I can't kill Louis Letrush? I did a quest with him involved and gained ownership of Frost. Before Frost died in an unfortunate accident (I am a bit wreckless with fire spells, lol) I noticed Letrush appeared next to Frost at the stables. I picked a fight with him several times and got his life bar to the last bit of red but he would never die.
Sorry if the above is a little "off topic". As far as the thread goes, I fall into the camp of wanting a role playing game to be just that. If you kill the wrong person or animal, tough, you pay the price. Obviously it can be painful at times if you miss out on the bounty from a quest but I like to play the game with a mind of "doing the right thing", albeit in a fantasy environment. I try to live my character with some of the traits I have personally and only enter fights when attacked. I may be missing out on something having lost Frost and J'zargo but I will have to live with the consequences of the decision I made.
Essential means the NPC is unkillable, so yes, because you were able to kill J'Zargo means hes not essential.
Most if not all followers have you perform a quest before they will follow you and so would would have done his and as such you will not miss out on any content.
Essential means that the NPC is essential to the game and therefore cannot be killed. They just crouch, but do not die, unlike for instance a follower who is crouching and gets hit by stray damage will die. Letrush is indeed a good example, its not a bug. He is classed as essential by the game and therefore cannot be killed.
There are some quests involved with NPC's that can die, and if they do so before the quest you will miss out on it.
But all of these quests are miscellanious so you will not miss out on any main quest content, or any other 'important stuff'.
Some NPC's are essential until a specific point in a quest or until the story of a quest requires them to die.
This is one of the reasons for essential NPC's. If a person is supposed to be killed at the end of a quest, but got eaten by a dragon some time before, that could mess things up a bit.
I would really like to see NPC's that are only killable by express actions of the player, and none that are just immortal, but I dont know if its technically possible.