Apart from such rumours that "Boo" ate raw squirrels and peeped into people's windows at night, he is ironically first depicted as a typical teenage boy: perhaps running around with the wrong gang and giving into peer pressure, but "normal" just the same. One has the impression that the more extravagent rumours came after his father's sanction for Boo's joy-riding in a "borrowed" car and locking up a school official in an outhouse. Social isolation stigmatizes Boo more than any prank or even crime he could have done; he becomes a pariah and a byword in the Maycomb community.
Instead of lashing out in rebellion (a normal reaction under the circumstances) as most boys would have done, Boo retreated into a world of his own, complying with the family rules to keep him "out of sight, out of mind." This is not only an unjust and demeasured punishment; it is a denegation of his right to even exist. Moreover, his invisible "presence" and the abnormality of his father's arrangement with local officials feed the rumours circulating about him. The other boys in his gang who went to a vocational school as part of their "rehabilitation" got on with their lives whereas Boo's life stopped in its tracks the day he disappeared into the Radley house as a permanent recluse.
Also on pages 9-10, Scout describes Arthur Radley in great detail:
". . .a malevolent phantom. . . .People said he went out at night when the moon was down and peeped in windows. . . .Any stealthy small crimes committed in Maycomb were his work. . . .Once. . .peoples' chickens household pets were found mutilated; although the culprit was Crazy Addie. . .people still looked at the Radley Place, unwilling to discard their initial suspicions. . . .Radley pecans would kill you. A baseball hit into the Radley yard was a lost ball, no questions asked."