in scout's eyes, what is atticus's chief fault/

In Scout's eyes Atticus chief fault in "To Kill a Mockingbird" is actually related to the issue of race.  Atticus is a defense lawyer appointed to defend a black man (Tom Robinson) in the novel.  From the start of the trial, the Finch family has been ostracized from many of the residents in Maycomb because Atticus set's out to discover the "truth" in the trial.   He defends Tom with such passion and conviction.  Many residents in the town, feel that Atticus has turned his back on his own race (especially when Atticus puts Tom on the stand and he states "he felt sorry" for Mayella a white woman.  The prosecution felt insulted by Tom's remark.

 Scout suffer's redicule from her own family, because of Atticus' duty to defend Tom.  Even Scout's cousin Francis taunts Scout about Atticus defending a black man and she curses and beats him up. 

   She is punished for her altercation with Francis.  At first, she blames Atticus, for the difficult position she believes he puts the family in.  However, she respects and admires her father toward the end of story.  After she tells her father and her Uncle (Frances father) that Francis actually called Atticus a "N lover"...her Uncle apologizes for punishing her. 

    The relationship between Scout and Atticus is endearing.  Atticus does a great job of prepairing his children for a world that is often cruel, and he teaches them the importance of tolerance.