This is in chapter 24 during the tea party.
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I think the intense irony is because Aunt Alexandra has ladies over for a meeting of the Missionary Society of Maycomb, and keeps Scout in attendance in order for her to learn to be a lady. The discussion moves toward the topic of Tom's wife, Helen. Apparently the black cooks and field hands in town were discontented during the week after the trial. One of the ladies comments on how much she dislikes a, "sulky darky," and says that when her black female servant was slow to perform her duties following the trial, she reminded her that Jesus never complained. Another lady says that no amount of education will ever make "Christians" out of black people, and that, "there's no lady safe in her bed these nights."
Suddenly Atticus enters the house and requests Aunt Alexandra and Calpurnia's presence in the kitchen. He reveals that Tom tried to escape from prison and was shot to death by the prison guards. Apparently the guards tried to tell him to stop and fired warning shots, but Tom kept running. The intense irony shows through here. THe women at tea and their ignorance is amplified by Atticus's dignified and sombre announcement.