Mr. Underwood compares Tom Robinson’s death to the senseless killing of a songbird with a simile or symbol.
Tom Robinson is Atticus Finch’s black defendant. He is accused of raping a white woman, but in Alabama in the Great Depression, it is difficult to get a fair trial. He becomes the mockingbird, symbolically, who is killed in the title of the book. Lee uses a simile, a literary device in which you make a comparison with the words "like" or "as."
Symbolically, Tom Robinson is a mockingbird because he means no harm and is only helpful. Atticus tells his children earlier that they should never shoot mockingbirds. When they ask Miss Maudie about this, she explains why.
“Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.” (Ch. 10)
What Atticus is trying to tell his children is that certain creatures do not harm others, and you should not harm them. He uses the mockingbird as a metaphor, because it is an innocent bird.
Tom Robinson is the innocent man. Atticus does his best to show this in court. He demonstrates how Robinson did his best to help Mayella Ewell, because he felt sorry for her, and how he meant no harm. Unfortunately, he was targeted by his society because he was a black man trying to help a white woman, and this was something that Maycomb did not appreciate.
This is essentially what Mr. Underwood was saying in his editorial after Tom Robinson is shot trying to escape from jail. He was not afraid to stand up for Tom Robinson, even though he was a black man, because he felt that it was wrong for the guards to shoot Tom Robinson because he was crippled.
Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. 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)
Although we do not have Mr. Underwood's exact words, we can assume he used a simile, and said something "like the senseless slaughter of songbirds" since Scout uses the word likened.
Tom Robinson escaped because he felt like he was trapped, and there was no hope of freedom. Even though Atticus was willing to try an appeal, Robinson did not feel that it would be any more successful than the first trial. The only way out for him was essentially suicide. He knew he was not getting over that fence, but he went because he could not stand to be locked up anymore.
Symbolically, Tom Robinson is a mockingbird because he is killed for no reason. He was doing no harm, and only trying to help. Atticus might have been thinking about him when he told his children not to shoot mockingbirds. Later, Scout also says that making Boo Radley a target is like shooting a mockingbird. Both men are innocents made targets by their society, and both of them are symbolic mockingbirds.