How does Mr.Underwood, in his editorial on Tom Robinson's death, evoke the symbol of the mockingbird?(Chapters 24-25)

Mr. Underwood compares the killing of Tom Robinson, who
was handicapped with one useless arm, to the killing of an innocent bird.  Even though
Robinson escaped (after the ridiculous and predictable guilty verdict of his trial), it
was still cruel and unnecessary to shoot someone who clearly wasn't going to get very
far.  This editorial evokes the same sentiment in the stern admonition Atticus had given
his children when they received their BB guns for Christmas:  "Shoot all the bluejays
you want, if you can hit 'em, but remember it's a sin to kill a mockingbird."  He went
on to explain that bluejays were pests that destroyed crops, but that all a mockingbird
ever did was to sing.