Mr. Underwood, in a fiery judgment against the shooting of Tom Robinson as he escaped, declares that "it was a sin to kill cripples, be they sitting, standing, or escaping." He likened Tom's death to the senseless slaughter of songbirds by hunters and children. He is basically saying that it doesn't matter that Tom was escaping; he was handicapped, and to shoot him was an awful and senseless atrocity, just as killing songbirds for sport is an atrocity.
This relates to the title, because earlier in the book Atticus had told the children that to shoot the Mockingbird was "a sin". To shoot an innocent bird, for no reason, that was just singing its song, was an awful sin. In essence, that is what happened to Tom-not only in his death, but also in his unfair conviction.
In Chapter 25, Scout mentions that Mr. Underwood wrote in his editorial that he felt that it was a sin to kill cripples. Mr. Underwood likened Tom's death to the senseless slaughter of songbirds. Mr. Underwood is essentially saying that Tom was a helpless, innocent man who did not deserve to be killed. Earlier in the novel, Atticus tells the children that it is a sin to kill a mockingbird. Miss Maudie elaborates by mentioning that mockingbirds do nothing to harm anybody, which is why it is considered a sin to shoot them. Mockingbirds symbolize innocent beings throughout the novel. Tom Robinson is a symbolic mockingbird. Similar to a mockingbird, Tom does nothing to harm anybody and even helps Mayella out with her chores. Unfortunately, Tom is crippled and defenseless in front of the prejudiced community. Killing Tom was similar to killing a harmless mockingbird. Atticus's instructions to not kill mockingbirds can be applied to protecting and respecting innocent people, which directly relates to the title of the book.