what leads to the narrator to that conclusion?

Atticus maintained that people, especially his children, should refrain from judging people until they experienced life from their perspective. By the end of the novel Scout had figured this out first hand. Scout had finally seen Boo Radley for who he really was. Boo was not a monster or a joke but her own guardian angel, her benevolent phantom. Scout was able to see why Boo was so reclusive and how they had all misjudged him.