How is Boo Radley from Harper Lee's To Kill a Mockingbird present as a menacing or threatening character?

Harper Lee's To Kill a Mockingbird is about a family living through the Depression in the fictional town of Maycomb, Alabama. It is something of a coming-of-age story in which the narrator, six-year-old Scout Finch, learns about the realities of life from her father, Atticus Finch, and a host of other local characters.

One of the most important characters in the book is the enigmatic recluse Boo Radley. This character is shrouded in mystery. This mysteriousness inspires Scout and her companions, brother Jem and friend (and six-year-old fiancee) Dill, to create an elaborate fantasy world with Boo at the center as a terrifying spectre of evil.

Lee ominously introduces the reader to Boo in the book's second paragraph:

. . . it began the summer Dill came to us, when Dill first gave us the idea of making Boo Radley come out.

But before that happens, Boo is the subject of many imaginary activities for Scout and the others. They talk about him incessantly, making up games that include running up to Boo's door, touching it and running away, sneaking into his yard at night, and pretending to stab each other with scissors (as Boo was alleged to have done to his father years before). They characterize him as a predator, believing, or at least pretending to believe, that he sneaks out at night and eats cats and squirrels.

It is doubtful that the real Boo is anything like that; in fact, the kids on several occasions find gifts in the knothole of a nearby tree that they suspect have been left by Boo. And the fact that they want to make him “come out” suggests that they are more curious than terrified by him. Boo is a way for the children to exercise their imaginations and also a way for the author to explore the ideas of prejudice and narrow-mindedness. The real terror in the book is the way people sometimes treat each other. The town's reaction to alleged rapist Tom Robinson mirrors the children's attitude toward Boo. As they see the injustice perpetrated by closed-minded, hateful people upon the victimized black defendant, they also come to see their own mistreatment of Boo.

Ironically, by the story's end Boo is the hero that saves Scout and Jem from a real enemy—Mr. Ewell, the story's epitome of ignorance and hate.