Miss Maudie describes Boo's (Arthur's) father, Mr. Radley, as a "foot-washin' Baptist-" which is to say he was incredibly strict. She says that such radically religious types believe that any pleasure is a sin. Miss Maudie is under the impression that Boo's father had been so strict in his religious devotion that Arthur's childhood was horribly affected and this resulted in his becoming drastically anti-social. If Mr. Radley was indeed that strict, he would have punished Arthur at every pleasurable moment. This is certainly not a recipe for a pleasant childhood. When Scout asks Miss Maudie if she thinks Arthur is crazy, she responds:
If he’s not he should be by now. The things that happen to people we never really know. What happens in houses behind closed doors, what secrets- (Chapter 6)
Note that Miss Maudie never calls him Boo; she always refers to him as Arthur. She won't condescend to call him names. Perhaps this is an indication that she doesn't blame Arthur for his reclusive behavior. Had she though Arthur was a bad person, she might be inclined to call him Boo. She recalls a time when Arthur was younger, "He always spoke nicely to me, no matter what folks said he did." Miss Maudie definitely wonders what Mr. Radley might have done during Arthur's childhood that would have affected him so dramatically. Considering that Arthur was, at one time, a nice young boy and that he became a recluse, Miss Maudie supposes that the mean-spirited, radically strict Mr. Radley had something (or everything) to do with the way Arthur turned out.