Tom Robinson is convicted, despite all evidence pointing to his innocence. Although Atticus wants to appeal the unjust verdict, Tom understandably has little faith in the legal system. He attempts to flee and is killed.
Atticus tells what happened in Chapter 24:
"They shot him," said Atticus. "He was running. It was during the excercise period. They said he just broke into a blind raving charge at the fence and started climbing over. Right in front of them....the guards called to him to stop. They fired a few shots in the air, then to kill. They got him just as he went over the fence. They said if he'd had two good arms he'd have made it, he was moving that fast. Seventeen bullet holes in him. They didn't have to shoot him that much."
Tom's disillusionment is shared by both Scout and Jem, who were certain until this point, that their father, and justice, would prevail.