It's not letter B 'because of slavery' it's letter B because Atticus is explaining that in no circumstance is white men take advantage of a black person okay
In this excerpt, Atticus is trying to prepare Jem and Scout for the racism that they will witness as they grow up. However, he does not want them to become racists themselves. Therefore, Atticus tells them that they will start to notice that white people take advantage of black people all the time. However, he thinks the people who do so are trash. The thing that makes these white people trash is the fact that they take advantage of black people.
