Why does Dill start crying in the courtroom in "To Kill a Mockingbird"?

The sensitive Dill cries in the courtroom because Mr. Gilmer, the attorney for the prosecution, questions Tom in a such a cruel and devious manner that it clearly indicates his racial bias.

When Tom Robinson is on the stand, Atticus asks him questions in the same manner in which he speaks to the Ewells or anyone else. However, when Mr. Gilmer takes his turn at interrogation, he abruptly begins without addressing Tom, and he simply calls him "Robinson" at the end of his question:

"You were given thirty days once for disorderly conduct, Robinson?" asked Mr. Gilmer.
"Yes suh."
"What'd the n****r look like when you got through with him?"
"He beat me, Mr. Gilmer."

After this disrespectful questioning, Mr. Gilmer badgers Tom with insulting questions, insinuating remarks, and demeaning epithets, such as "boy":

"Had your eye on her [Mayella] a long time, hadn't you, boy?"

Then, when Tom naively says that he felt sorry for Mayella, Mr. Gilmer purposely twists the meaning of Robinson's words by inflection to make them sound as though Tom feels himself superior to Mayella--a serious social error in the Jim Crow South:

"You felt sorry for her, you felt sorry for her?"

No one among those seated on the ground floor likes Tom Robinson's response, especially after Mr. Gilmer stresses certain words. Too late, Tom realizes the implications of what he has innocently stated. After this, he becomes very nervous as Mr. Gilmer persists with his near-badgering of the witness. It is then that Dill starts to cry, and is unable to stop. When his sobs become audible, Reverend Sykes suggests that the children go downstairs and leave the building.
Once outside, Dill explains that he is sick of Mr. Gilmer and his "hateful" way of talking to Tom. When Scout tells Dill it is Mr. Gilmer's job to cross-examine, Dill counters,

"I know all that, Scout. It was the way he said it made me sick, plain sick." [sic]...."The way that man called him 'boy' all the time and sneered at him, an' looked around at the jury every time he answered--...Hasn't anybody got any business talkin' like that--it just makes me sick."

This is one of those cases in which the old phrase "out of the mouth of babes" comes to mind. That is, it takes the innocent perception of a child sometimes to perceive wrongs that others do not. Thus, the sensitive Dill intuitively realizes how vituperative and disparaging Mr. Gilmer is toward the innocent Tom Robinson, one of the "mockingbirds" of Harper Lee's novel. Obviously, Gilmer has Tom already condemned in his own mind, and he hopes to sway the jury to condemn him, too, as he glances at this jury meaningfully while he questions Tom. Furthermore, he slants the tenor of Tom's answers by putting his own interpretations upon them through the use of inflection and tone as he repeats them.