It wasn't a devil contract though. It was literally just the (religious) belief that drinking any amount of alcohol would eternally damn both the drinker and their family.
Given that's literally what I said in response to it--that you do yourself no favors with such an example--it's nice that we've finally reached agreement.
Really? There was nothing hyperbolic in the thing you were responding to. The person you responded to was speaking about how you as GM decided that's what this character would believe, so appealing to "realism" isn't a defense. As was said just a bit upthread, "realism" isn't causing that character to have that belief. The GM is causing it.
Would it help to tell you I don't think that?
Instead, I think that GMs need to EARN trust. Nobody deserves trust. It must be earned. One of the most effective and efficient means to gain trust is to be cooperative, straightforward, and forthright. Playing by rules, and making those rules clear and check-able by all participants is another highly effective means to gain trust. Conversely, being secretive, exercising power without possibility of oversight, and rejecting rules as "too confining" is...not how one gains trust. Worse, doing those things and then saying, "Well why don't you just trust me?"--because then you've made the very act of asking for accountability and openness an offense, and you've staked the requirement that the player always and infinitely trust the GM no matter what actions they might take.
Er...how, exactly? I laid out in very specific terms why this temptation is there, what incentives motivate it, and why GMs would be likely to overlook the (extreme) deleterious consequences. That isn't in any way "vague platitudes" like "I try to run a realistic setting".