The Radleys have been neighbors of the Finch's since before Jem and Scout were born. The children have heard the stories and rumors about Boo for years:
People said he existed, but Jem and I had never seen him.
None of the stories are good. Boo is a "malevolent phantom," a ghoul who kills, mutilates and eats animals raw. Boo poisons pecans, peeps in windows at night, and freezes azaleas simply by breathing on them. He is known to have stabbed his father in the leg with a pair of scissors, and Jem believes Mr. Radley must keep Boo chained to the bed to prevent him from escaping from the house that has become his personal prison. The fear of the unknown is a powerful thing, and had the children ever seen Boo, they might have discovered that at least some of the stories were untrue. But with Miss Stephanie routinely providing the latest rumors and accusations about Boo, Jem and Scout grew up believing what the children and adults said about him. Like the Negroes who "would not pass the Radley Place at night," Jem and Scout always ran past the Radley house as fast as they could.
Jem and Scout only feared Boo, because of how they felt about him. they thought that he was some creepy guy that stayed in the house and wanted to kill people and .... in reality, he was actually a nice guy that just commited a crime when a teenager, so his dad made him stay in the house.