As Jorb posted before, animals are not supposed to kill you after KO'ing you, other than the rare situation where they have another attacked queued up. This is evident to me, as I've been aggro'd by a badger while KO'd and it immediately deaggro'd and walked off.
However, this behavior does not always happen, due to these situations:
1. An animal will queue up an attack instantly after hitting you, meaning it will hit you again (Possibly kill)
2. You and a friend have aggro'd an animal, and it has switched aggros at least twice, it will attack you, then attack you until you die as it does not register that you are KO'd
3. Multiple animals have aggro'd you and one did not twitch in time to deaggro you after knocking you down, meaning it will continue to fight you as if you were standing
I don't know if Jorb wants to necessarily remove the "mercy" from animals (I think it'd be lame). But at minimum, a method for us to gauge whether an animal will be incredibly dangerous to us in a situation (I figure Raging bears should always kill. Why would it turn peaceful if it's mad?) instead of relying on chance/luck for the animal to not twitch at a wrong moment and end up killing you. This also applies since it will kill you even if getting beat by your friend (As the aggro doesn't switch in time).
I myself and a friend have died to this bug a few times, and I think I've narrowed the cause slightly for point number 2
It requires an animal to switch targets twice.
Let's say you aggro a boar, and then lead it around for a friend to aggro. If the boar switches to your friend but they become incapable of chasing (Climb a cliff) and then the boar aggros you again, the boar will down you + continue to beat you until you die (It hit me 3 times total, it was not a queued attack).
It also happens if the boar KO's an enemy after it switches away from you, and then it will hunt you and kill you, due to being stuck in combat (And an aggressive state) with your friend.