At this point in the novel Beatty has found Guy's secret out about the books. Montag's wife has turned on him and turned him in to Beatty- the fire chief knows about his books. Beatty confronts Montag about the books and tells him he must do the job himself. It's bad enough to own books, but it's even worse that fireman, whose job it is to burn books, has his own book collection. Beatty doesn't just want Montag to burn the books, he wants him to burn everything piece by piece so that he learns his lesson. That's the point of the quote- Beatty wants Montag to repent by burning everything himself. By doing this he can prove to the department that he's wrong. Beatty is forcing Montag to take responsibility for breaking the law.