A story is not finished, until it has taken the worst turn.
Eine Geschichte ist dann zu Ende gedacht, wenn sie ihre schlimmstmögliche Wendung genommen hat.
Original text (German)
Notes from Wikiquote
As translated by James Kirkup; also quoted in CurtainUp Reviews, Williamstown Theatre Festival (Summer 2007) by Elyse Sommer
Variants
A story is thought through to the end when it has taken its worst possible turn.