Do your players even bother to try and talk to folks they meet in dungeons?
In my game the players have met a lot of people in dungeons. It started out well, but I think they've come to the conclusion that you need to just kill what you meet.
- they met two men-at-arms in the hobgoblin caves. They turned out okay (and Korric and Orrie might make a comeback, who knows)
- they rescued Raggi Ragnarsson the berserker from some gnolls.
So far, so good. But then, Felltower.
- They met the Choke brothers. Heheheh. Sorry, that's still funny. Five throttlers, who tried to kill them. They got one of the PCs.
- they met a crafty gnome, believed his lies, and they've been stumbling across his vicious traps ever since.
- they rescued a petrified hobgoblin, recruited him, and then beat the survivors of a hobgoblin tribe into working from them. They didn't last.
That's about it for meet-and-greet people in the dungeon. Not a lot, really. But couple that with the various creatures that try to kill them, they've started to think they need to just kill who they meet. They've made good on that, and killed at least two other monster-types that would have happily negotiated with them or made a deal with them without even making an attempt. When they do negotiate, they tend to work from a point of "you capitulate and we'll see if we're willing to give you anything after that. Otherwise you die." Not a real incentive to talk to the PCs, really.
They may have a point, though. Most people, most critters, and in the dungeon with bad intentions. Direct attack at least takes advantage of your constant wariness and positioning. You're ready to fight immediately, and trying to make a deal inevitably gives the other guys a chance to stab you in the back or re-start a fight on better terms. Starting from a totally lopsided negotiating platform might work, and if not, you wanted a fight anyway. It's a good assumption that violence will occur, and often both sides just skip right to the violence. Especially since I've made it clear that officially, what happens in the dungeon is flat-out legal. Fighting has a real cost in resources and potentially in lives (a bad roll in any fight can kill your guy), but getting jumped by "allies" has a potentially worse cost, and attacking everything puts it on a cost basis you are best prepared to pay. You're coming ready and able to inflict violence.
I'm sure they'll just attack the next guys to try to talk to them.
That tells me it's time to make sure there are a few more upright folks who'd honor their deals, just to make sure it's always a potential loss to fight everything you meet. If my ratio of backstabbers to frontstabbers to potential friends is off by too much, there really is no point to try and talk to anyone. There must be a cost to not talking.
And the players will be right: never trust a dungeon dweller.