What is the theme in Sweeney Todd: The Demon Barber of Fleet Street by Hugh Wheeler?

The play's theme of corruption is announced by Sweeney in the first scene when he describes London as "a great black pit" inhabited by "the vermin of the world." There is no morality in Sweeney's London where "at the top of the hole / Sit the privileged few / Turning beauty into filth and greed."