In Chapter 29 of To Kill a Mockingbird, we see that Jem and Scout have been attacked by Bob Ewell and saved by someone. The children walked home alone after the Halloween pageant at school and Bob Ewell followed them. He attacked them and broke Jem's arm. Bob was going to try to kill Scout, but some unseen person stepped in and saved the children. When Atticus and Aunt Alexandra find out about what happened and who was behind it, Aunt Alexandra feels like she could have somehow stopped it.
"Atticus, I had a feeling about this tonight - I -this is my fault," she began, "I should have-"
Aunt Alexandra had heard of Bob Ewell's threats against Atticus and others who had anything to do with the trial of Tom Robinson. Of course, no one ever thought Bob would come after Jem and Scout, yet everyone knew what kind of man Bob Ewell was. He beat his daughter, Mayella, and bullied many people in the town. Atticus was the one man he couldn't bully, so he decided to go after what mattered most to Atticus: his children. Aunt Alexandra carries around guilt, thinking she could have stopped this in some way. She loves Jem and Scout, and feels like she should have done something. We all know that she couldn't have stopped Bob Ewell. He was a nasty man set on revenge. There was no way anyone could have known that Bob Ewell would have gone after Jem and Scout. Luckily the children were saved by the person they spent so much time trying to see.