Well, the end of the story is the biggest instance of black humor when she serves the policemen the murder weapon for dinner (the leg of lamb with which, while frozen, she hit her husband over the head and killed him). That is also dramatic irony because her husband himself was a policeman, and you would think that she would be the primary suspect, like it would happen in a real situation where there are no other witnesses that could appeal to the contrary.
In a situation similar to the story I would think that she snapped. She was pregnant, happy to be married, and he was totally nonchalant about how told her that he was leaving her. He would have changed her life by turning it upside down. I think I would have snapped too. And blame it on hormones.