"Sweeney Todd," by Stephen Sondheim, is often ranked (among Broadway zealots) as the greatest American musical in our history. (Its big rival is "Gypsy," also by Stephen Sondheim.) One of the joys of "Sweeney" is the way it breaks some unwritten rules. For example, a rule: Your protagonist must be likeable. Nellie Forbush must be a ray of sun, a cockeyed optimist. Marian Paroo must be a fine sister, a fine daughter, a paragon of good sense. Maria the postulant misses mass because she wants to admire the "dark green shadows" of the trees. Who wouldn't love that problem called Maria? But Sweeney Todd is gloomy; he is a half-zombie. A plan he has is to commit mass murder. He doesn't "warm the heart." Second, another rule: The hero should get what he or she wants. Nellie finds love on some enchanted evening. Marian discovers her white knight. Maria weds and (I guess) she climbs an erotic mountain, among other mountains, in Act Two...