Q1

From the text:

"Boo was about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch, that's why his hands were bloodstained -- if you ate an animal raw, you could never wash the blood off. There was a long jagged scar that ran across his face; what teeth he had were yelow  and rotten his eyes popped, and he drooled most of the time." ( 13)

The children believe that Boo roams around the neighborhood backyards at night.  

When the children hear scratching at night, they think that poossibly it's Boo out seeking revenge. They believed that he scratched on screens and tore them apart with his fingers.