Why is it significant that Mr Underwood describes Tom and death as the senseless slaughter of songbirds in his editorial?

the theme of how killing a mockingbird is a sin is how it relates. mocking bird's do nothing but make lovely music- as described similarily by ms maudie, so to kill them would be senseless. by killing mockingbirds you are destroying innocence and as tom was innocent, his death was like the senseless slaughter of mockingbirds- or song birds.