Mr. Underwood says that it is a sin to kill cripples, which is similar to Atticus saying it is a sin to kill mockingbirds, making Tom Robinson a symbolic mockingbird.
The title of the book comes from Atticus telling his children not to shoot mockingbirds because they are harmless. What he is really telling them is that a mockingbird is a gentle creature that deserves respect, because he does only good for others, and does not do anything against others.
"I'd rather you shot at tin cans in the back yard, but I know you'll go after birds. Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird." (Ch. 10)
Mr. Underwood writes in the Maycomb paper a scathing editorial in response to Tom Robinson’s death after he is convicted and shot trying to escape. He is upset that anyone would shoot Tom when he is obviously incapable of defending himself.
Mr. Underwood didn't talk about miscarriages of justice, he was writing so children could understand. Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. (Ch. 25)
The connection to the title continues when Mr. Underwood “likened Tom's death to the senseless slaughter of songbirds by hunters and children,” basically saying that killing Tom was like killing a mockingbird. Tom Robinson is a metaphorical mockingbird.
Tom is an innocent man who did nothing wrong, and he is twice wronged by the legal system. He is convicted in court simply because he is black, and then he is shot in prison. True, he was trying to escape, but Mr. Underwood’s point is that he was shot because he was black. He was harmless, because he was a cripple, and it was completely unnecessary. It was racism through and through.