I have recently finished the civil war quests siding on the imperials.

Earlier, in Winterhiold I tried killing a npc. It was the nord insullting the dark elves in the grey quarter. He wouldn't die so I assumed he was marked essential for some reason like a quest.

Now, I came across a stormcloak camp. They attacked me and I killed them except for one, a general with a name. He wouldn't die as I think he was marked essential.

My question, are all npc with names essential? And even though the civil war quests are over will I never be able to kill named generals in stormcloak camps?

Thank you. I appreciate any input.

Not all of them, just the ones essential to major questlines (generals, Jarls, etc.).

Take Whiterun for example, you can kill pretty much everyone- including named, BUT the Jarl and General.