How does tom robinsons death make him a metaaphorical mockingbird?

In his editorial, he discussed the sinfulness of killing Tom Robinson. It was a sin, just like it was a sin To Kill a Mockingbird ( theme of the story ) because he was a cripple. He already had a disadvantage, so why kick him while he's down already. It also discussed of the outcome the jury and how it decided to pose Mr.Robinson guilty because of his race, and that Atticus tried his best, and all that he could for Tom Robinson not to lose his respect, and rescue him from being jailed away. It was Maycomb's fault that he was put there, while knowing he was innocent. So they sinned, like it is a sin To Kill a Mockingbird. It is shameful.
"Mr Underwood wasn't writing about justice, he was writing so that children could understand. Mr Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom's death to the senseless slaughter of Songbirds by hunters and children, and Maycomb thought he was trying to write an edititorial poetical enough to to be reprinted in The Montgomery Tribune."
Mr. Underwood likens Tom's death to "the senseless slaughter of Songbirds," an obvious reference to the novel's title.