Here's how it goes: a critical hit (natural 20) that doesn't kill your target has a chance (1% per fighter level) of becoming a deadly blow. A deadly blow multiplies your damage by 1d20.
A 8th level fighter has 8% chance of dealing a deadly blow for each natural 20. It will hardly come up against lowly enemies (they die immediately most of the times), but occasionally, when fighting a dragon or dinosaur, you'll create the scenes you read about in Conan stories (I'm thinking Red Nails) or Tolkien (Bard versus Smaug).
And now even a punch (or improvised weapon) could kill.
Of course, fighting a dragon becomes a lot more dangerous, for PCs of ANY level. No more guarantees...
(If you want armor so give you a bigger chance of survival - which I think is fair - subtract 1 point from the d20 for each point of AC better than unarmored, to a minimum multiplier of x2).