At the beginning of the story, the three kids have heard all kind of frightening stories about Boo and come to believe in them, perhaps to escape the monotony of their own lives. The fact that Boo never left his house is another reason for the children to be afraid of him.
As the story continues, the reader sees some good actions from Boo: leaving presents for the children in the tree, covering Scout with a blanket after the fire, fixing Atticus's trousers. It is because of these actions that the reader can begin to understand the nature of Boo.