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Roald Dahl's "Lamb to the Slaughter" most noteworthy themes reflect the conflict between fantasy and reality that the character of Mary Malone displays throughout the story.The fantasy lies on Mary's acquired identity of wife, future mother, and happy homemaker. She has made of her home a middle class utopia. Yet, the reader (and apparently Mary) does not know what is really happening within the marriage. Has Patrick been displaying obvious signs of dissatisfaction that Mary has chosen to ignore? Is this an actually happy marriage? Is abuse being atoned for with superficial bliss? Hence, fantasy versus reality is the theme that serves as the background to other conflicting sub-themes.
Another of those themes is love versus submissiveness. Mary Malone's passivity and willingness to please are synonymous with the ideal middle class life; a wife stays home, serves the husband, and tends to the house. Her reward would be her husband's utter satisfaction, as well as his income, as he is the sole provider and head of household.
However, in Mary's passivity seems to coincide with a secretly non-altruistic desire of being validated with praise or demonstrations of love. Judging by the behavior of Patrick Maloney, Mary is likely to lack that validation.
A second thematic conflict is loyalty versus betrayal. In Patrick's actions we see coldness of heart as well as of character; how can a man leave his wife when she is six months pregnant with his own first child? When he perpetrates his betrayal, her loyalty seems amplified; Mary even insists on getting dinner ready as usual. Yet, it is here when she snaps, kills him, and then has to figure out how to cover her actions.
One last theme is justice versus injustice: A man has been murdered by his wife, seemingly in a moment of insanity, and because of what he does to her. Is it fair that Mary should be home free just because she knew how to make the weapon disappear? Is it fair that she was left by her husband? Is it fair to put her in jail with no previous record and a six month pregnancy? All these conflicting questions are part of the theme, and the answers only come to the reader depending on their own schema and opinion of Mary and Patrick.