I get that this is a harsh world, everyone is fighting for survival, not much space for the meek. However, if EVERYONE is trying to cheat you, then mistrusting becomes an easy choice. So, the suggestion. Doesn't need to be for the demo, it can be in the next areas... Why not add one or two innocent encounters, where trusting someone is the right thing to do? Why not add an honest and innocent character, even if a minor one? This will leave the players a bit off-guard, since they're not expecting it, and the contrast between this small candle of innocence and the brutal world around it will only reinforce how everyone else is crooked and jaded. It's a matter of contrast.