what was the author's purpose by using this

I think there is a definite tone shift in chapter 21. The trial has been eventful with a lot of suspense. Atticus has proven beyond a shadow of a doubt that Tom Robinson is innocent. The tone however turns sad and self-conscious when Tom is still found guilty by the jury. The jury knows that they have fallen on the side of bigotry and hate: in their hearts they are ashamed. Jem is crying and Atticus is clearly disappointed and disillusioned.