It is documented fact, from multiple video games where players have the ability to choose whether to do good or evil, they overwhelmingly break toward good. Even in things where it's expected that you play through it multiple times, and thus (for example) achievement statistics should eventually regress toward most players at least attempting both paths, the overwhelming majority only choose the "good" path, at least as the game in question defines it.
Humans aren't nearly the bastards you portray them to be. Yeah, we often suck, and trusting that truly not one person will do bad things is a recipe for disaster. But it is certainly closer to correct than assuming that all or almost all people are puppy-kicking, bloodthirsty thieves and murderers. Even in D&D, where our fantasies can come to life, people (believe it or not!) often fantasize about doing right by others.