How did Mr. Underwood, in his editorial on Tom Robinson's death, evoke the symbol of the mockingbird?

    Although the character of newspaper editor Braxton Bragg Underwood does not otherwise fit the bill as one of the "mockingbirds" in the Harper Lee novel, To Kill a Mockingbird, his editorial following the death of Tom Robinson does more closely resemble this idea of innocence and purity.
    Underwood, who supposedly was not a lover of Negroes, had silently defended Atticus at the jail when the lynch party showed up to take Tom Robinson away. Now, following Robinson's death, Underwood's editorial borrowed from Atticus's own saying that "it was a sin to kill a mockingbird."

... Mr. Underwood didn't write about miscarriages of justice, he was writing so children could understand. Mr. Underwood simply figured it was a sin to kill cripples... He likened Tom's death to the senseless slaughter of songbirds by hunters and children...

It was a "senseless killing." Atticus had proven without a doubt that Tom could not have been the killer, but

... in the secret courts of men's hearts, Atticus had no case. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed.