Boo Radley never leaves his house. He has a troubled past. If you follow his story closely, you will see he is not actually a bad person, but he just had some differences of opinion with his parents and his brother and a difficult childhood.
Boo Radley definitely had a history of violence. He was a juvenile delinquent who ran around with a gang who was up to no good. The other boys were sent to state industrial school, but Boo was released to his parents. He was later arrested for attacking his father.
According to Miss Stephanie, Boo was sitting in the living-room cutting some items from The Maycomb Tribune to paste in his scrapbook. His father entered the room. As Mr. Radley passed by, Boo drove the scissors into his parent’s leg, pulled them out, wiped them on his pants, and resumed his activities (Chapter 1).
Those inicents explain from where some of the legends of Boo Radley developed. Boo's exact motivations are unclear. His parents were very religious, and Boo felt they were keeping him locked up. He seems to have snapped.
After his arrest and release, Boo never left the house. The children began interacting with him, and there are subtle indications of his eccentricity. The first is that you can hear laughter when they are trying to make Boo leave his house. Does he enjoy the children’s game? Then, the children find gifts in a tree hollow. These are small objects like pennies, gum, and twine, but the most unusual is the soap dolls.
They were almost perfect miniatures of two children. The boy had on shorts, and a shock of soapy hair fell to his eyebrows. I looked up at Jem. A point of straight brown hair kicked downwards from his part. I had never noticed it before. Jem looked from the girl-doll to me. The girl-doll wore bangs. So did I.
Boo Radley was trying to reach out to two children in his neighborhood without coming out of his house. This sounds very odd, and maybe even inappropriate. Given that the interactions are all childlike and innocent, though, we have to remember Boo Radley is really just quiet and shy. The Finch children and Dill are the only people who have ever taken any interest in him.
By watching Scout, Jem, and Dill, Boo Radley is able to live vicariously through the children without exiting the house. That is to say, he rarely comes out of the house. The only times he comes out, Boo is trying to interact with them. He leaves them presents, sews Jem's pants, and puts a blanket on Scout's shoulder.