Mary Maloney kills her husband in a sudden burst of rage because he has told her he is going to leave her at a time when she is six months pregnant with their first child. She is a very loving, devoted, domestic wife and is understandably outraged at his betrayal. Her husband does not tell her that he is involved with another woman, but there seems to be a good possibility that such is the case, which would add jealousy to her motivation. She is an example of the old saying, "Hell hath no fury like a woman scorned." This became a popular folk saying, but it was used originally by William Congreve (1670-1729) in his play The Mourning Bride:
Heaven has no rage like love to hatred turned,
Nor hell a fury like a woman scorned.
One of the most interesting things about Roald Dahl's story is seeing the mild-mannered, demure housewife suddenly and spontaneous turn into a murderess and then go on to save herself from punishment by concocting an alibi and destroying the murder weapon in an ingenious way.