According to Atticus, the prosecutor's case was based on " ...an evil assumption...". Please explain his quote. please help me(:

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.