As most of us remember in Morrowind, every single NPC was able to be killed, albeit at the cost of the integrity of the main storyline: With this character's death, the thread of prophecy is severed. Restore a saved game to restore the weave of fate, or persist in the doomed world you have created.
When Oblivion and Skyrim rolled around, this mechanic was replaced with invincibility for your vital NPC's to the story. If we're really going to harken back to the Morrowind days- Shouldn't we give fan-service, and make it so that nobody may live forever?

Probably not 100% (for
Probably not 100% (for technical reasons), but most NPC you will be able to kill.