I've noticed that sometimes I am able to kill essential NPCs. Today, for example, while playing I was able to kill Legate Taurinus Duilis (Morthal). When this happens though I always get the kill camera (a 'special' move). Is this a thing or is my game just glitching out?
I haven't completed the Civil War quest, on this save I've practically not touched the quests involving either the Stormcloaks or the Imperials. I was able to kill Farengar too, though it took a few tries, the first few times he 'crouched' like essential NPCs do when fully damaged but I was eventually able to kill him. I do have the Unofficial Patch though.