It depends on how much damige they take. My NPC goes down if she takes too much damige but still has helth left. But if she gets nailed (hit hard). Or if you do what I did and accidentally nail her with unrelenting force while taking out six other bad guys. Then an NPC can die without kneeling.
Followers can die. So far the only way I've seen it happen is if I accidentally hit them myself for the final blow, or if they are hit with a poisoned weapon at low health.