What does Atticus mean when he says the prosecutor's case was based on "an evil assumption" in To Kill a Mockingbird by Harper Lee?
In chapter 20, when Atticus gives his closing remarks in the Tom Robinson case, he wants to let everyone know this case is unfair and evil. A trial like this should never have gone to court because the prosecution should have had the burden of proof, but they did not have any proof to substantiate even holding a trial. The only "evidence" the prosecution brought forward was two white people's lying testimonies against a black man's. Atticus breaks down all of the facts as best he can during the trial, so, by the time he makes his closing remarks, he has a chance to let the jury and the community know just how twisted the trial and society are.
Atticus says that the testimonies of the the Ewells are highly doubted based on the facts and the lack of evidence the prosecution presented. The prosecution brought the case to trial for one reason and that is based on "the evil assumption—that all Negroes lie, that all Negroes are basically immoral beings, that all Negro men are not to be trusted around our women" (204). Atticus is basically saying that white people in Maycomb seem to believe that anything associated with black people is either a lie or bad. It's as if white people do not believe a black person could ever be innocent of a crime if a white man charges him with having committed one. This is the evil and completely false assumption that the community and the prosecution need to abandon. Anyone can lie, be immoral, or not be trustworthy, not just black people.
The rest of Atticus's statement explains what the "evil assumption" is. This is toward the end of chapter 20. He goes on to say that the assumption people in Maycomb are making is that all Negroes lie, all Negroes are basically immoral beings, and that all Negro men are not to be trusted around white women. Furthermore he says that this is an assumption associated with minds of "their caliber", meaning minds like those of the Ewells. In the next paragraph he says that the assumption is a lie. While some Negroes lie, some Negroes are immoral, and some Negroes are not to be trusted around any woman, it is a truth that applies to the human race, not to a particular race.