Why was Boo Radley a bad guy in the beggining of the story and turns out to be a good guy in the end?

In the beginning of the story, Boo is sort of the local freak show. He is a virtual hermit, subject of ghost stories concocted by neighbourhood gossip and children. By the end of the story, Boo becomes a guardian angel. He follows Scout and Jem from the evening pageant. When that nasty Bob Ewell attacks the children, Boo saves them.