Why does Dill get "sick"? Who else understands?

Dill is sickened by the way Mr. Gilmer talks to and about Tom. Dill feels that he demeans him, even if he isn't sure he understands what it all means.... he knows it isn't right.

“Well, Mr. Finch didn’t act that way to Mayella and old man Ewell when he crossexamined them. The way that man called him ‘boy’ all the time an‘ sneered at him, an’ looked around at the jury every time he answered—”