Harper Lee shows in Atticus' speech how much the human being tends to be unfair, ignorant, and maintain a negative hierarchy between people.
Atticus' final speech occurs when he fails to defend an innocent man and deliver him from death, even with proof of his innocence. Given this fact, he shows that it is human nature to be unfair when it comes to situations involving people of low hierarchical levels in society.
This makes him point out that the human being is biased and that he ignores the truth if it doesn't benefit him.
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