I would go out on a limb and say that Ralph's confidence in humanity's decision to do the right thing is his tragic flaw. Ralph is a good leader, and he is selected by his peers to be the leader over Jack. Ralph is more fair and noble than Jack, but he does expect people to pull their own share of the weight as far as work goes. He expects that the signal fire will be kept going. He expects that fresh water will be collected. He expects that food will be collected. He expects that every tribe member will act as he is expected to act for the greater good of the tribe as a whole until rescue occurs.
He does not take into consideration the darkness of human nature that lurks just under the surface and leaps as the chance to be free. Ralph does not anticipate Jack's style of leadership which incorporates manipulation and fear tactics. Ralph certainly never saw the murders of Simon and Piggy coming. He simply had confidence that every boy on the island would do what was right and good.