Of course, many players dislike the sense of losing control over their characters. I'm the one playing this character, why are my actions being dictated by circumstances beyond my control? If I decide that my character is a heroic martyr and would give up his or her last sip of water, who's to tell me otherwise?
The reason for this set of rules isn't just to take away control from players or simply to underscore the brutal realities of the desert world of Athas (though that latter is certainly part of the equation). This rule exists to create terrible situations in which characters do awful things to their allies so that they can regret it later!
SPOILER ALERT! If you've seen the movie Snowpiercer, you may recall the scene in which the hero describes having descended to cannibalism in order to survive. It's a pretty rough scene, it's a little overplayed, but it does bring together a bunch of elements previously seen in the movie, and like the rest of the movie it creates a junction between metaphors and physical elements. For the character at least it's a moment of powerful admission of a dark past and a troubled time of wondering, "Can I really do this? I'm a monster, how can anyone trust me to make a better world?"
The water desperation rules in DARK SUN fit this role for characters -- they create a circumstance in which characters who otherwise are reasonable people will descend to horrible, desperate measures to survive, so that they can regret those acts later and dwell on the terrible things they have done. Throw characters into one of these situations, watch them shank each other for the last dregs of a dirty waterskin, and then watch the echoes of guilt on the faces of the survivors later.
Now, naturally, you never want to run a straight-up "My character is going to beat-down the party" scene unless all of the players agree to it; otherwise it just turns into a contest of rules mastery and who can be the biggest jerk. If players agree to these kinds of scenes, though, you can get great drama out of them, not just in the moment, but in the form of lasting repercussions about broken trust and shattered ideals.
Other kinds of dark secrets are quite possible (and likely!) in DARK SUN, too; doing horrible things goes hand-in-hand with survival in the blasted world. Why would any player want these kinds of awful secrets and past elements? For the drama and the experience, of course! Having horrible secrets in your past -- or even non-secret terrible deeds -- is a great springboard for spotlight time ("Time for me to talk about my lament of the terrible choices I made!"), for reflecting on similarities to the current situation ("I made this mistake once before, never again!"), and for earning bonus experience points for role-playing through these tribulations.
If you don't want all of your dark secrets to revolve around who you killed to get the last bit of water during a desperate journey in the desert, here are some other possibilities to give to your character. You don't need to have any kind of dark secret, but no dark past means no duress, which means one less opportunity for beefy role-playing and experience rewards!
Note that the Dark Past ideas here might be cases in which your character actually did something awful, or maybe (s)he just feels guilty for having survived a random horrible event and thinks that (s)he was responsible. This table is also great for important NPCs that accompany the party -- at a tense moment the party may suddenly discover that the noble psion, diligent cleric, or altruistic preserver is not the person they thought they knew!
DARK PAST (1d20)
- Turned in a member of your own family to the templars in order to save yourself
- Murdered a stranger for his/her last few bits
- Stole money/food from orphans/cripples/beggars
- Sold a friend out to slavers
- Gave up a traveling companion to thri-kreen/halflings (who presumably ate said companion)
- Betrayed a secret elemental cult and its local priest who were helping the community
- Betrayed a community group that was standing up against local criminal elements
- Promised to help someone escape from slavery/prison and then failed to follow through
- Promised to bring help to someone who was dying and then failed to follow through
- Promised to deliver a heirloom/message, but lost/sold it instead
- Deserted from a military group/local militia that was making a desperate stand
- Took advantage of someone's romantic affection to get something, then let the person down
- Showed a defiler where to find a place of unspoiled nature
- Stole a secret technique from a craftsman/professional/artisan
- Spread slanderouos lies about an innocent person, who then failed to perform an important altruistic task or committed suicide
- Destroyed a priceless item that was the hope of many people
- Let raiders/gith/slavers/bard (assassin) in through a back door to a vulnerable community
- Betrayed test of trust to an elven band
- Preyed upon children as easy victims for selling to slavers/selling to thri-kreen/dinner
- Shot the food