delericho
Legend
How many times can an entire party of killers be defeated an captured before it becomes silly?
Of course, back in Middle Ages warfare, nobles tended not to die, because if they were captured then their relatives would pay a handsome ransom. If the PCs had an agent in town handling their money, the same model could be used - do the evil cult just kill them, or do they take them captive, ransom them back, and thus use the money to expand?
(Which of course has a couple of other benefits, too. It serves to soak up some of that excess money they've been acquiring across their careers. And it gives them a great motive for future adventures - they now have good reason to seek revenge.)
I think it's just mathematics. You can capture them occasionally (but players tend to find that unfun and the "hah! you were only unconscious! bazinga!" thing is gonna get old pretty quick) and you can kill them once.
In general, I take the view that if the party fights until their all unconscious, the DM should absolutely show no mercy. Fight to that point, and you're dead.
But this is still supposing just two outcomes - either the PCs win or they fight to unconsciousness (or death). What happened to cutting their losses and running away? Or a negotiated surrender when it becomes obvious they're overmatched?
If the players know that there is a good possibility that they'll find themselves overmatched at times, and especially if it happens reasonably often, surely they would adapt their playstyle accordingly?
(Plus, I blame video games, where if you fight the BBEG and lose, you just load from a saved game and do-over. D&D generally doesn't have this safety valve, so players expect the game to adjust so they can always win.)