When Aunt Alexandra says that Ewell could do something furtive to Atticus, she means that Atticus should be very careful. She warns him that Ewell will be very secretive.
Aunt Alexandra is suggesting that Bob Ewell might try to harm Atticus in a secretive or sneaky way as revenge for defending Tom Robinson in court. She is concerned about the potential danger that Ewell poses to her family.
In chapter 28 of "To Kill a Mockingbird," Aunt Alexandra has a premonition about Bob Ewell seeking revenge on Atticus and his family after the trial. She worries that Bob Ewell, being humiliated by the trial's outcome, may try to harm them.
The conflict between Bob Ewell and Atticus in "To Kill a Mockingbird" mainly stems from Ewell's resentment towards Atticus for defending Tom Robinson in court. Ewell feels humiliated by the trial's outcome and takes his anger out on Atticus and his family. Atticus remains calm and tries to defuse the situation through his unwavering sense of justice and integrity.
Mr. Ewell spits in Atticus's face, threatens him, and later attacks his children, Scout and Jem.
Mayella Ewell wanted Atticus to dismiss Calpurnia because she felt that Calpurnia was getting too close to Tom Robinson. Mayella feared that Calpurnia would influence Atticus to see the truth of Tom's innocence.
In the courthouse, Mr. Ewell confronted Atticus about being threatened during the trial. Atticus remained calm and assured Mr. Ewell that he had no intention of harming him. Mr. Ewell eventually left, feeling humiliated.
When Ewell was on the stand, Atticus asked him to write his name. Signing your name is a habit; you'll always use your dominant hand. Ewell complied, not realizing why Atticus asked for this.
Bob Ewell is angry with Atticus because Atticus exposed him as a liar during the trial and defended Tom Robinson, a black man who Bob accused of attacking his daughter. Atticus's actions challenged Bob's credibility and social status in the community, leading to his resentment towards Atticus.
Bob Ewell spits in Atticus' face out of spite and a desire for revenge, as he is angry that Atticus was able to prove his daughter Mayella was lying about the assault. Ewell's pride is wounded, and he seeks to humiliate Atticus as a form of retaliation.
Atticus suggests that Mayella's father, Bob Ewell, beat her. He mentions during the trial that it was likely Bob Ewell who harmed Mayella out of anger and frustration, rather than Tom Robinson.
Bob Ewell had falsely accused Tom Robinson of raping his daughter, Mayella Ewell, which led to Atticus defending Tom in court. This created tension and animosity towards Atticus and his family in their small town.
Bob Ewell vows to get even with Atticus Finch because Atticus embarrassed him during the trial and made him look foolish. Bob feels humiliated and wants to seek revenge on Atticus for challenging his credibility and exposing his lies.
Mr. Ewell spat in Atticus Finch's face and threatened him after the trial of Tom Robinson in "To Kill a Mockingbird."