The mere thought or mention of the courthouse brings up Jem's bad memories of the Tom Robinson trial: The unfair jury verdict, Atticus's unsuccessful defense, and Tom's eventual death. At the end of Chapter 26, Jem goes from being the kind and helpful brother who is willing to drop what he is doing to answer Scout's question to, at a moment's notice, being
... suddenly furious. He leaped off the bed, grabbed me by the collar and shook me.
Scout did not seem to understand Jem's actions, so she sought out Atticus. Her father explained that Jem was just trying to forget, but the wise Atticus saw something else in his son. According to Atticus,
... what he was really doing was storing it away for a while, until enough time passed. Then he would be able to think about it and sort things out. When he was able to think about it, Jem would be himself again.
Because Jem has knowledge of the law and some of the methods of lawyers, he has followed the proceedings of the trial of Tom Robinson quite well. Then, because of the illogical turn of the verdict, Jem is confused and the world that he has known up until now is disturbed.
Having his organized and logical world upturned, Jem becomes unaccountably angry. When, for instance, Scout tries to tell him about what Mrs. Gates said as she came out of the courthouse, Jem explodes,
"I never want to hear about that courthouse again, ever, ever, you hear me? You hear me? Don't you ever say one word to me about it again, you hear? Now go on!" (Ch. 26)
Because the proceedings of the trial have become such a travesty of justice, Jem is upset and cannot make any sense of the terrible outcome. An innocent man has been convicted of a crime he has not committed on purely circumstantial evidence and the fabricated testimony of irreputable people. And now this innocent man is dead for no reason that is logical, at all.