It's a little more complicated than that. Brod (who it would be more accurate to characterize as Kafka's close friend than a purely professional publisher) did

It's a little more complicated than that. Brod (who it would be more accurate to characterize as Kafka's close friend than a purely professional publisher) did not promise to destroy the works; on the contrary he made clear that he would do no such thing. It has been suggested that by leaving his works to Brod, Kafka intended that they should not be destroyed, despite his words contradicting this. (Kafka had a complex relationship with his own writing and so this ambivalence was not out of character.)