Why does Scout consider Boo to be a monster in To Kill a Mockingbird?

Scout has lived near the Radley home her entire life. The
house is creepy enough to her, but she has heard rumors as long as she can remember
about Arthur Radley, who she has never seen. His nickname to everyone in Maycomb is
"Boo," presumably for his ghost-like persona. Boo is only seen at night (and very rarely
at that), creeping around in the shadows. He is accused of every unsolved crime that
occurs in Maycomb, and Miss Stephanie Crawford claims that he peeks in her windows after
dark. Worse of all are the rumors of Boo's bloodthirstiness. Wild animals and pets are
in danger when Boo is around, and Jem claims that he has been known to eat squirrels
raw. The fact that Boo may be mentally unstable is another cause for concern among the
children. Even the truthful tales about Boo--that he stabbed his father with a pair of
scissors, for example--are unnerving to Scout. Of course, all of this changes once Scout
and Jem begin receiving gifts in the secret knothole. They soon come to realize that Boo
is no monster, but a friendly, though mysterious and invisible,
neighbor.