To Kill a Mockingbird wasn't one of the required readings for my HS, so I don't have a copy of it handy to help you here and it's been forever since I've read it.
I'm totally available to try and help you if you'd like, though.
Anyways, I pulled up some articles. Radley was the introverted man who preferred to remain in seclusion inside his house, right? Most articles I'm reading describe him as the quintessential mockingbird of the story - mockingbirds are presented in the book as a symbol of innocence and purity of heart, and Boo's
self-imposed exile is in fact a result of his dissatisfaction with the corruption of the outside world around him. The positivity in the title refers to the fact that his courageous actions in saving the children are covered up by the townspeople because publicizing them would put Boo out of his comfort zone. He's a hermit, and he only stepped out of his comfort zone to help the children based on his moral principles; thus, to flaunt the virtuosity of his actions would go against his quiet, modest nature. According to Finch, to "kill a mockingbird" is a crime - and by leaving Boo alone in peace, they're protecting him.