How does the way Mr. Underwood describes Tom's death relate to the title of the book To Kill a Mockingbird?

Mr. Underwood says that it is a
sin to kill cripples, which is similar tosaying 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 Robinsons 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. Underwoods 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.