Mr. Underwood compares Tom Robinson’s death to children senselessly killing birds.
When Helen Robinson hears the news of her husband’s death, she does not react at all. She falls to the ground, and Dill compares it to a child squashing a bug in the dirt.
"[She] just fell down in the dirt. Just fell down in the dirt, like a giant with a big foot just came along and stepped on her. Just ump-" Dill's fat foot hit the ground. "Like you'd step on an ant." (Ch. 25)
This is related to Mr. Underwood’s scathing comparison of Tom’s shooting children shooting birds, just as the title of the book suggests.
He likened Tom's death to the senseless slaughter of songbirds by hunters and children, and Maycomb thought he was trying to write an editorial poetical enough to be reprinted in The Montgomery Advertiser. (Ch. 25)
This is a reference to Atticus’s comment earlier that Scout and Jem should not shoot mockingbirds with the guns they got for Christmas, because all they do is make sweet music that people like to listen to. Underwood, and therefore Lee, is reminding us that Robinson is a mockingbird. He was an innocent who was wrongly targeted for someone else’s benefit.
Helen’s inability to respond to her husband’s death is more than just a showing of grief. It demonstrates how she has been treated by society. She is the bug. Just as children play with bugs and squish the roly-poly bugs for no reason, because they can, there is literally nothing she can do about her husband’s death. She and her husband are helpless. Society will not help them. Although she is grieving for her husband, she also feels the pain of knowing that she is helpless.
One of the main themes of the story is that the people that society throws away are the ones that it should value most. Tom Robinson was a kind man, who took pity on a young girl and tried to help her when no one else would. Like the other mockingbird in the story, Boo Radley, he is disenfranchised and helpless, but society is better off for having him in it. The people of Maycomb who look out for them, like Atticus, Mr. Underwood, and the children, cannot help them alone. It will take more than a good closing argument and a scathing editorial to bring about change.