In chapter 25, Mr. Underwood's article hits right at the title of the novel itself, because he said that killing Tom Robinsons was like "the senseless slaughter of songbirds" for mere entertainment. He asserted that people killed Tom just because they could, and because it was the way that things were done (speaking of racism). Scout reads the article and has two reactions; the first one is her initial, confused reaction. She doesn't understand why Mr. Underwood was so upset, because Tom had a fair trial, was convicted by the jury, and had gone to jail and tried to escape, then was killed in that escape attempt. It was sad, yes, but everything had been done the way it was supposed to be done. But then, as she thinks on it more, she realizes that Mr. Underwood was right--Tom had never had a fair trial, and the odds had been stacked against him. She realizes that
"in the secret courts of men's hearts Atticus had no case. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed."
Unfortunately, the situation was one that was, in their society and time period, a fatal one for Tom. Any black man accused of raping a white woman, no matter how much the evidence vindicated him, was condemned in a society that was racist and ignorant, and afraid to go against the status quo. The minute Tom was implicated in the crime, he was a goner--there was no way around it, unfortunately. Scout grows up a little in this moment, realizing the rather unfortunate and sad side of human nature that exists in her town. I hope that those thoughts helped; good luck!
Mr. Underwood's article, written in response to the shooting of Tom Robinson in the jail yard, likens the man's death to "the senseless slaughter of songbirds by hunters and children." (I'm pretty sure that's an exact quotation from To Kill a Mockingbird, but I don't have the novel on hand to be sure.) Scout here experiences the realization that Tom was unjustly killed (at the trial, I believe, she was already convinced of his innocence), and she has a similar realization in the final chapters of the novel, where she states that exposing Boo Radley to a legal inquiry concerning the death of Bob Ewell would be like "shootin' a mockingbird." I like the novel, but this theme of the destruction of innocent lives is sometimes treated in the novel in a manner that seems a little heavy handed.