i already finished the civil war questline on the imperial side, and afterwards, I was ordered by legate rikke and general tulius to clear out all of the stormcloak camps across skyrim. In my travels, i've come across many of these camps but every time i do, i have the same issue: the leader of the camp won't die. I can kill all of the other soldiers fine but when it gets to the leader, he just kneels down wounded when his health goes down, but then he just gets up with all his health is back. No matter how much I slash, arrow, and fire spells at him, he won't die. anybody know how to fix this?

Nope, they are essential. No idea why though. Actually being able to go and capture camps and eliminate enemy generals would have made a lot of sense.

I have the same problem, strange seeing as I've completed the questline for the empire.

Do you even get quests for them? Because in the empire questline all the quests I got were off Legate Rikke

I've ran into this issue as well.

I believe this was an oversight by the development team. What should have happened was that those NPC's should have had their "essential" tags removed after you finish the last quest on either side of the civil war.

This is just another one of those small bugs that I hope gets addressed when Bethesda starts rolling out those Data fix patches for all platforms come January.