Both stories deal with the idea of "the perfect crime" committed by a woman. But whereas Mrs Maloney's act of striking her husband dead is not premeditated, the twinkle-eyed middle-aged landlady had done same thing twice before and is "at it" again. She is definitely "off her rocker" in a Norman Bates kind of way (as in "Psycho") whereas Mrs Maloney just seems to have momentarily lost control. However, Mrs Maloney giggles while the policemen are discussing the eventual murder weapon "right under their noses,"revealing the perverse side to her personality. She also keeps her cool directly after the murder, going to the store and getting things for supper, even a desert, "to please" Patrick (and to offer herself an alibi). Her deliberation and calculation are somehow incoherent with her sentimental and romantic nature. This paradox adds to the complexity and "scariness" of the character.
Roald Dahl wrote a third story using the same idea of the female criminal committing the perfect crime. In "The Way Up to Heaven" Mrs Foster, the perfectly servile housewife driven to distraction, takes advantage of the circumstances of only a few seconds to settle accounts with her husband once and for all. A "must" if you have enjoyed these first two tales.
In terms of similarities between the two stories, both perpetrators are women. Both are described as benign and not, supposedly, able to do anyone any harm. Mary Maloney, in "Lamb to the Slaughter," is described as calm and peaceful, with large eyes which emphasize her innocence. The landlady, in the story of the same name, is defined as having a "warm welcoming smile,"a pleasant look, and as someone who "seemed terribly nice."
Furthermore, the two are both responsible for committing capital crimes -- taking the life of another. They both get away with their criminal acts and, seemingly, have committed the perfect crime. In addition, it seems that both wrongdoers derive pleasure from their criminal deeds and possess an inherent malice. Mary laughs at the end of the story ("And in the other room, Mary Maloney began to laugh"), whilst the landlady seems to look forward to the idea of preserving her victim when she says, “I stuff all my little pets myself when they pass away." This is, essentially, where the similarities end.
There are many differences between the two. In the first place, the plots are dissimilar. Mary is a pregnant young woman awaiting the arrival of a much-loved spouse who she kills after hearing that he is going to leave her. She creates an alibi and calls the police, whose investigation does not turn up any definite clues. They, at Mary's insistence, eat the evidence, a cooked leg of lamb which, when frozen, was used as the murder weapon. In the end, Mary laughs when she realizes she is going to get away with murder.
The Landlady, in contrast, relates the story of a middle-aged woman who runs a Bed and Breakfast boarding house. She lives alone and intentionally murders a select number of guests by poisoning them with cyanide (hence the almond smell in the tea). Her latest guest is a seventeen year old, Billy Weaver, who falls for her charm. She systematically sets him up by extending warmth and kindness so that he may unsuspectingly drink his tea, which she laced with arsenic.
Furthermore, the reader knows exactly what Mary Maloney did, since the narrator tells us. In "The Landlady," however, the crime is suggested through a journey of discovery by the new lodger and the landlady's responses to his questions and her actions, such as when she looks him up and down and mentions that her previous tenants are still with her. The references to the stuffed animals also provide clues to her intent. The reader has to infer what she is up to.
In addition, Mary Maloney did not intentionally set out to kill Patrick, her husband. Her actions were not deliberate. In "The Landlady," however, the perpetrator deliberately plans her crime and follows a meticulous routine. There is no need for a cover-up, for it appears that she would never be a suspect. Mary Maloney, though, has to create an alibi, which she does with aplomb.
Another contrast between the two stories is the mood. In "Lamb to the Slaughter," the mood is less foreboding or tense. Mary, although purportedly sad at the beginning, regains her mood and speaks freely to the investigators. In "The Landlady," however, the reader senses the tension and build-up to the malicious malefactor's final act.
Finally, one could also argue that the stories are contrasts, since the main character in one (Mary) can claim extenuating circumstances for her crime while, such mitigation is absent in "The Landlady."