As Matti-patti said, you can insta-kill enemies, and the rules aren't different for the player and the enemies. In practice it doesn't really matter whether your character dies instantly or gets knocked unconscious and an enemy finishes you off while unconscious, though.
I remember on character being lost after firing shot after shot at a Nerp, missing everyone, and then have him firing a single arrow back, blood welling up from the chest, and everything go dark.
Another character failed to hit a wolf with a javelin throw (at 75% effective skill), miss a spear attack (same skill), and then a dodge (again, roughly same skill), get bitten in the arm and faint from pain, never to wake up again (while the still leashed dog obviously didn't manage to prevent death).
The first injury above was probably lethal, while the second one definitely wasn't (the second character had had a history of fainting when getting injured, including being hauled away by robbers with a whole posse of companions on the scene, still there when the character returned with the fight continuing where it was broken off).