In Chapter 19 of To Kill a Mockingbird, discuss Dill's reaction to Tom's cross examination by Mr. Gilmer.
Dill is literally sickened by the manner in which the prosecutor, Horace Gilmer, questions Tom Robinson. Gilmer repeatedly addressed Tom as "boy," treating him as disrespectfully as possible. Like Jem and Scout, Dill had previously sided with Atticus before the trial, but he has also seen for himself through the testimony that Tom is an innocent man. Part of Dill's reaction is because he has never been in a courtroom--Jem and Scout have seen Atticus in action before--and he is taken aback by the prosecutor's persistence. Scout explains that it is just part of his job, but Dill is upset because Tom is being treated differently because he is a Negro.
"It ain't right, somehow it ain't right to do 'em that way. Hasn't anybody got any business talkin' like that--it just makes me sick."
Dill soon finds that he has an ally outside under the oak tree in Dolphus Raymond, who is also sickened by the way African-Americans are treated by white people.