Only the player can finish her forever. Mobs, traps, and anything besides the player just brings her to her knees. But the player can easily one-shot her when she's like that, killing her. Even with Unrelenting Force, I think.
So, lots of people kill her inadvertently.
Not true. Not true at ALL.
Lydia was killed by a Dragon execution move in my game and she wasn't even on her knees. She just got the living [censored] chewed out of her and tossed to the side like a stale bag of chips.
It was at that moment that I realized followers were not immortal. Since her, I have gone through 4 other companions who have died entirely from mobs. Brenor is the only one I've gotten that feels immortal; though he did die a few times (though I reloaded) while doing the investigation portion of the Markarth/Forsworn quest in the Treasury.