Why is Atticus powerless in To Kill a Mockingbird?

While Atticus acknowledges the coexistence of good and evil in the same person; an example being with Mrs Dubose, he underestimates the extent of Bob Ewell's vemgefulness. After Ewell harasses Judge Taylor, Helen Robinson and even spits on Atticus' face, telling him that he will kill him "even if it took the rest of his life" Atticus does not worry too much, telling that he had "destroyed his last shed of credibilty at that trial...";"so if spitting in my face and threatening me saved Mayella Ewell one extra beating, that's something I'll gladly take. He had to take it out on somebody and I'd rather it be me than that houseful of children out there."

However, a drunken Ewell assaults Jem and Scout near the end of the novel and they were saved by Boo Radley. Jem's arm got broken, and Scout would have been killed had she not worn the ham costume. Aunt Alexandra blames the incident on herself, telling Atticus that she "had a feeling about tonight"; that she should have gone to the pageant with Scout and Jem. Therefore in the novel "To Kill a Mockingbird" Atticus is a "mockingbird", to an extent.