This letter is sad because it describes a woman who is a terrible wife. The saddest part is that the writer admits at the end that he allowed her to became who she is and therefore, destroy his household. The writer's words make it sound like she controls everything in the house. He and the rest of the family are victims being oppressed by a female dictator.
He describes her as "jealous" and says he is tired of her "overbearing aggressiveness." Then he says this about himself; "anxiety cuts so deeply that I can hardly bear to go on living. My rage is so great that I often forget the calamaties I might cause." To me, he is a coward because he has held his feelings in so long that he is ready to explode.
That is so sad.