Although Scout and Jem had never seen Boo Radley, Boo had certainly been keeping a watch on them. Some of the neighborhood rumors about Boo were probably true: He did occasionally creep around at night, since he had made contact with the children before the assault by Bob Ewell (placing the blanket on Scout's shoulders and by mending Jem's lost pants). Boo had initiated the leaving of gifts in the knothole, and he watched them from within the Radley house as the children rolled the tire and played their Boo Radley game on the sidewalk.
Boo was the children's neighbor, friend and protector--Jem and Scout were "his children," too--although it took Scout a long time to recognize this. On that fateful Halloween night, Boo may have seen the children leave alone and unescorted, and decided to keep an eye on them. Boo may or may not have even known about the Tom Robinson trial or Bob's threats against Atticus, but then again, Boo was one surprising fellow.
Boo was our neighbor...
Summer, and he watched his children's heart break. Autumn again, and Boo's children needed him. (Chapter 31)
As was mentioned in the previous post, Boo viewed Jem and Scout as his friends. He quietly watched them play from his home and even attempted to communicate with them by leaving gifts in the knothole of the tree. On the evening of Bob Ewell's attack, Boo was more than likely watching Jem and Scout walk the streets alone. Like any concerned neighbor, Boo recognized Bob Ewell as a threat to the children and intervened when Bob attacked. Boo had the children's safety in mind when he left his home to fight off the attacker. Despite the fact that Boo considered Jem and Scout to be his friends, Boo was simply acting like a concerned citizen would if they witnessed two children being attacked by an adult. Boo understood that Jem and Scout needed help and selflessly came to their aid. Although Boo is reclusive and does not say much, he obviously knew that the children were in danger and decided to intervene. Personally, I do not think his appreciation for Atticus or his affinity for the children played a major part in his decision to intervene. Fortunately, Boo was able to prevent Bob Ewell from extensively harming or killing the children.
Because he is not as bad as he seemed, and had started to like the kids after the number of times they had met, even if it was at a window. He is a nice person and any self respecting person would have done the same.