Ralph really doesn't make a mistake in believing in the goodness in humans. Ralph is the rational, logical, and adult representative on the island (other than Piggy, of course). Ralph keeps trying to organize the boys so that they can maintain a signal fire, build huts to survive the elements, and maintain a cohesive group. Ralph keeps doing so even after Jack's band of hunters grows and even after Simon and Piggy are "murdered."
Part of the appeal of Lord of the Flies is that it presents versions of humans in their various "states." Ralph is a leader fashioned on goodness, logic, rationality, and reason. Jack is representative of wildness, action, impulse, and immediate gratification. All of the other characters could be similarly placed on such a spectrum. Ralph makes no mistakes--he is a representative of "good" in the novel.
In "Lord of the Flies," Ralph recognizes the value of the order of society which can provide the structure that is necessary for the boys to remain civilized. His mistake is that he does not realize that not everyone is reasonable, and it is this reason that Ralph equates with the goodness of humanity. For, in the final chapter as the hunted Ralph kneels in the shadow, feeling "his isolation bitterly," he reflects, "They were savages it was true; but they were human...."
Perhaps Ralph's greatest flaw is that he fails to recognize the power of fear into which evil creeps. This mistake is exemplified in the final meeting that Ralph calls. For, when he becomes angry, the boys give him respect out of their fear. However, as Golding mentions, a "cloud" comes over Ralph's mind at times and when one boy acts silly and the others laugh, Ralph loses his control of the group. It is then that Jack usurps the power, shouting "Bullocks to the rules! We're strong--we hunt!" Jack's assertion of strength appeals to the weakness and fears of the boys who then turn their respect to him. The assembly "shredded away and became a....random scatter." Even Ralph has fear at this point, shuddering violently, wondering, "Are there ghosts, Piggy? Or beasts?"
Jack's evil, charismatic personality sweeps the boys under his control. With the help of the sadistic Roger, boys such as SamnEric are controlled and made to join the hunters against Ralph as Jack leads the way across the bridge of rock, a bridge that connects the boys' primal fears with their primal nature of savagery, "the darkness of men's hearts."