The passage that you are looking for is a couple pages into chapter 25. Mr. Underwood writes an editorial after Tom Robinson was shot and killed as he tried to escape from jail. In this editorial, he compares Tom Robinson to a beautiful songbird that is shot down purely for the trophy, the sport, and the pride of having shot it. It's a pretty extreme comparison, and one of the parts of the books that relates directly to the title of the book itself. Mr. Underwood specifically
"likened Tom’s death to the senseless slaughter of songbirds by hunters and children"
speaking pretty directly about the nature of the people that shot him, potentially risking losing readers and advertisers, but he was so upset that
"he couldn't have cared less who canceled advertisers and subscriptions."
Basically, to interpret his meaning, he meant that Tom was shot down merely because people could. They didn't try to just wound him, or capture him, they unnecessarily shot him to death, not even thinking about whether or not the man was innocent or not. This is just like songbirds; little kids shoot them with toy guns, and hunters shoot them, just because they can, not thinking for a moment how it is an innocent creature that has never done them any harm.
I hope that those thoughts help to clear things up for you a bit; good luck!
In Chapter 25, Scout mentions that Tom Robinson's death did not receive much press and few citizens were interested in it. However, Mr. Underwood wrote an editorial which likened Tom's death "to the senseless slaughter of songbirds." Mr. Underwood simply felt that it was a sin to kill cripples. After reading the article, Scout realizes that Atticus had no case in the "secret courts of men's hearts." Throughout the novel, mockingbirds symbolize innocent beings. Tom Robinson was a symbolic mockingbird because he caused no harm, tried his best to help others, and was defenseless against the ugly racism throughout Maycomb. It was painfully obvious that Tom Robinson could not have raped Mayella given his handicap and Mr. Underwood felt that it was a sin to harm defenseless beings. He compares Tom's death to the "senseless slaughter of songbirds" which draws the connection between Tom Robinson and innocent mockingbirds.