What does Mr. Underwood compare Tom's death to in To Kill a Mockingbird?

Mr. Underwood compares Tom
Robinsons death to children senselessly killing birds.

When
Helen Robinson hears the news of her husbands 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. Underwoods scathing comparison of Toms 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 s comment
earlier thatandshould 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 elses benefit.

Helens inability to respond to her husbands 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 husbands 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 mainof 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, , 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.