Whose death does mr. underwood compare to killing a mockingbird? how does this fit the explanation about why it is a sin to kill a mockingbird that miss maudie gave the kids?

Tom Robinson's death was the one compared to killing a mockingbird. This fits the explanation because Tom never did anything wrong, (other than everyday human stuff.) He died ultimately because he was put on trial for something he didn't do, and convicted because of the color of his skin.