What does this quote mean? from to kill a mockingbird?

I'm not a very good man, sir, but I am sheriff of Maycomb County. Lived in this town all my life an' I'm goin' on forty-three years old. Know everything that's happened here since before I was born. There's a black boy dead for no reason, and the man responsible for it's dead. Let the dead bury the dead this time, Mr. Finch. Let the dead bury the dead.