Boo Radley, as a metaphor for fear, works in two directions.
First, Boo Radley is a figure of fear and mystery for the children, representing an unknown entity that may be violent, insane, feral or otherwise wild, and vindictive. Second, Boo Radley is a frightened man when he finally appears in person, afraid to walk home by himself on a dark night.
Rumors and absence combine to make Boo Radley a special character in the life of the imagination of the neighborhood.
A local legend for several years, Boo is rumored to wander the neighborhood at night and dine on raw squirrels and cats.
These stories cannot be contradicted by fact because no one has seen Boo Radley for years. This allows Radley to become a specter, fitting in with childish notions of "haints" and "hot steams". The mystery that surrounds him is essentially what allows his persona to be imbued with fearful elements.
In person, however, Boo Radley is quiet and timid. He apparently has either developed a fear of the world while staying indoors all these years or was driven to remain in the house initially because of that fear. Whatever the cause of his fear may be, Radley manages to briefly overcome it to interact with Jem and Scout from a distance and finally to save them from Bob Ewell's attack.
Regarded as a freak by the whole town, judged for his unwillingness or inability to join in with public life, Boo nonetheless has the courage to keep on living and to reach out to the children.
Boo Radley is a symbol/metaphor of fear for kids in the book. Since nobody has seen him and nobody has met him, everybody takes the pleasure of describing him the way he wants. The whole incident where Radley killed his father with a pair of scissors is prejudiced against the reality. Associating everything else with that incident, the people relate him to all kind of creatures they see in horror movies.
As Jem says that he is Six and a half feet tall with blood washed yellow teeth. He says that though he has not seen him but people have seen such big foot prints and tracks around the house. The description that he feeds on dead squirrels and other animals, he drools all the times, his dark red eyes are popped out are all subject to the imagination of the kids.
Talking about Radley gives goosebumps to the kids and it is their favourite summer pass time. The facts that they shiver while talking about him, and are scared to walk past by his house during late evening hours end up tagging Radley as a metaphor for fear.