There are a few of ways to handle this really and one of the ways is discussed in another thread regarding Fate Points.
( http://www.ironcrown.com/ICEforums/index.php?topic=18097.0 )
At PC creation, I have my players roll 1D4.
1, 2, 3 = 1 Fate Point.
4 = 2 Fate Points.
These points can be used for anything a player wants to use them for, like reroll a death crit they received. It's Fate. They weren't meant to die now, they were meant to find this treasure, etc. I've had players use them for different purposes such as locating a secret opening they had open-ended-fumbled, another wanted to find a really great weapon in the treasure hoard. I personally don't allow players to re-earn Fate Points. The game is no fun if everyone is immortal and they can keep getting Fate Points to avoid death. That tends to players not putting much thought or creativity and... CAUTION into their gaming. Playing a game on God-Mode gets boring fast.
One caveat though. There's no guarantee the re-roll won't be as devastating as the first.
Something my group has done since I started playing is start all players at level 5. That gives players 6 levels worth of development, hopefully investment in more skills such as Body Damage Stabilize (a huge skill your players should have if your world is non-magical.... actually, your world sounds a lot like mine
), First Aid, Second Aid, Surgery, Herb Lore, Region Lore, Flora Lore, Use/Remove Poison, Poison Lore. These are all skills that players may not want to invest in at first level as they try to build up their PCs, but given 6 levels worth of development, PCs are more fleshed out and can handle more rigors of adventuring.
Another option that is always available to the GM... change the crit if you think it's just too early in the campaign to drop players. The players don't know what the GM rolled and they don't know what the crit is. It's your discretion.
Switch dice if you are killing off PCs too easily. Everyone has a pair of "These-damned-dice-are-cursed!" and they switch out for their "lucky dice." I know I do!
Finally... and most importantly....
PARRY PARRY PARRY!
Players tend to forget to parry, especially if they've been playing another game system where they can ONLY attack or ONLY parry in a given round. Taking 25OB and adding to DB can be the difference between 95E Crit and 95C Crit, or simply not even being hit.