Further to the other answers, there is a certain irony in our final view of the law and the legal process in To Kill a Mockingbird. Heck Tate, a very good sheriff to judge by what we see of him in the novel, decides to ignore orthodox procedure in investigating the killing of Bob Ewell and even removes crucial evidence from the crime scene. Atticus, the epitome of legal and moral integrity throughout the novel, is compelled to participate in the cover-up of Boo Radley's part in events despite his initial determination to follow due process. Of course we all feel that there is a natural justice in the way Heck Tate handles the finale, but it is an odd irony that, for all his principles about the law, Atticus' inaction at the end of the novel is, strictly speaking, a subversion of that law.
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.
Sheriff Tate knew that Tom Robinson was innocent and that it was only the town's extreme prejudice and Bob Ewell's testimony and finger pointing that caused Tom to be convicted of raping Mayella Ewell. He knew that Bob Ewell was the one who beat up Mayella and caused all the problems and now Bob Ewell is dead. It was Bob Ewell's fault that Tom was convicted and it was the conviction that caused Tom to run in panic. When he ran, he was shot and accidentally killed. Sheriff Tate is saying that the case was over now since Bob Ewell was dead.
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Because Tom Robinson, a young man, is dead because he just gave up, panicked, and ran. There was really no reason for this, because if he had waited it out, surely they would have won on appeal. It was such a clear case of not guilty, but the townsmen didn't have the moral courage to let an innocent black man go free, since that would make Bob Ewell out to be a liar and a child abuser. Bob Ewell lied during Tom Robinson's trial and it was his lies that ended up in a conviction, and then when Tom tried to escape and was shot, in a sense, Bob Ewell was responsible for his death, because Tom wouldn't have been in a place of such desperation if it hadn't been for Bob Ewell's lies.