Recently Philip Watson asked on Google+ how often we have evil NPCs join the party only to turn on the players at a critical moment. I said that I never do it because it trains players to behave in ways that I don’t enjoy.
Let me explain. Once players start mistrusting everybody it leads them to kill innocents because they fear betrayal, it leads them to torture prisoners because they might be lying, it leads them to endlessly debate whether this or that is trustworthy information, and so on.
Lies: I much prefer to be upfront about lying non-player characters. “The old man says this and that, but you’re suspicious. You feel he might be lying to you!”
Non-player characters are an important information channel. I can use them to tell my players about the world, to provide hints and help, I can have non-player characters agree with a plan if I think it’s good enough and they should stop planning, I can have non-player characters refuse to go along with a plan if I feel it is lacking an escape option, etc. Once I introduce untrustworthy information into this channel, it’s poisoned, effectively closed.
Promises: A similar idea is that my prisoners never lie. When prisoners beg for mercy and promise to run away and never oppose the party again, then that’s exactly what they do. Once released prisoners start getting reinforcements, I train my players to always kill prisoners.
Torture: In the current political situation I feel that torture is being downplayed and made palatable, it’s practically legitimized again. Where as the real world is sliding back into the dark ages, I refuse my game world to do the same. Thus, if my players want to torture somebody, I find that highly distasteful. To drive home how useless torture is to gain information, I usually ask my players “Ok, what do you want to hear?” And then, whatever they say: “That’s exactly what he says.” I refuse to role-play stuff I don’t want to read about in books.
If my players start to incredulously ask different questions, I might try and make my point again but it won’t take much for me to tell my players out-of-game that I really don’t want to hear about torture at my table ever again.