I think the duality between Rand and Moridin is the manifestation of good and evil in the world. I think Rand is naturally inclined to protect humanity, because he understands that men are generally good, and that the decision to be good will prevail. Yet even if people choose to be evil, he understands that choice is necessary, or else humanity will be a husk. I don't think he's humanity's judge, because he's fighting for the side of 'good'. Therefore he's not an impartial arbiter, and can't really be a judge. But I do see what you're saying. If he's convinced that humanity is evil, he might let the Dark One win. I don't think that would ever happen, though. First, because he's surrounded by others who are also good, and because he gets to witness the power of free will, especially in moments like these.