In "To Kill a Mockingbird," why does Tate say, "there's a black boy dead for no reason, and the man responsible is dead"?

That's his way of saying that he knows that Tom Robinson's death was unjust, even if he was found guilty via a trial, and that Bob Ewell's death, by contrast, was just, even if the courts never had a chance to rule on it.

 

By coming back to it, as he does in a number of ways, Tate is also letting Atticus know how firmly he's committed to his position. He's refusing to prosecute Boo Radley, period. In this Mr. Tate is showing that Atticus' efforts at trial have had some effect, and that he's not the only man willing to stand up for what is right: Mr. Tate is too.